Color image forming apparatus using registration marks

ABSTRACT

In an image forming apparatus for forming a color image by superposing component color images, a control device obtains position data representing a forming position of each of plural registration mark images for the component color images with respect to a positional reference value arbitrarily determined, divides the position data into each of a unit reference range representing a range in which one mark image is formed, converts the position data divided for each unit reference range into those based on respective reference values representing the front edge of the unit reference ranges, extracts the position data representing ranges common to the mutually overlapping ranges each of which is represented by a couple of position data corresponding to the rising and falling edges of a passage timing pulse as read position data of the mark image of the component color, and calculates the amounts of positional deviations of the component color images from one another on the basis of the position data extracted.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional application of U.S. applicationSer. No. 11/126,128 filed May 10, 2005, which is a Division of Ser. No.10/435,743, filed on May 8, 2003, now U.S. Pat. No. 6,911,993 whichclaims the benefit of priority of JP 2002-140431, filed on May 15, 2002,and JP 2002-166142, filed on Jun. 6, 2002.

BACKGROUND OF THE INVENTION

This invention relates to a color image forming apparatus such as atandem-type color printer or copying machine and a complex machine ofthese having an intermediate transfer belt or a photoreceptor belt andan image forming method suitably applied thereto.

In recent years, a tandem-type color printer or copying machine or acomplex machine of these has been used in many cases. In these colorimage forming apparatus, there are provided exposure means, developingdevices, and photoreceptor drums for the respective colors of yellow(Y), magenta (M), cyan (C), and black (BK), an intermediate transferbelt, and a fixing device.

For example, in the exposure means for the color Y, an electrostaticlatent image is formed on the photoreceptor drum on the basis ofarbitrary image information. In the developing device, a color image isformed through the deposition of toner particles of the color Y on theelectrostatic latent image formed on the photoreceptor. The toner imageis transferred from the photoreceptor drum onto the intermediatetransfer belt. As regards the other colors M, C, and BK, the sameprocessing is carried out. The color toner image, having beentransferred onto the intermediate transfer belt, is fixed by the fixingdevice.

Incidentally, by a color image forming apparatus of this kind, a colortoner image has to be formed on an intermediate transfer belt withoutcolor deviations (positional deviations of component color images fromone another, that is, component color images being out of registration).The reason is that the color toner image composed of component colorimages superposed without color deviations should be transferred onto apaper sheet.

FIG. 54 is a drawing showing an example of the detection of colorregistration marks according to a prior art. In FIG. 54, a processingcalled “color registration mark detection” is applied to theintermediate transfer belt 6 (also called the belt unit or paperconveyance belt) periodically or non-periodically before a color imageis formed on the basis of arbitrary image information. In this detectionprocessing, by the use of reflection-type photo-sensors (hereinafteralso referred to as registration sensors) 12A or 12B, etc., the“7-shaped” color registration marks (marks made up of a horizontalstraight line segment and an oblique straight line segment meetingtogether at a point with an acute angle, and hereinafter referred toalso as color registration marks CR simply) are detected.

At this time, the light, having emitted from the photo-sensor 12A or thelike, is intercepted by the color registration mark CR, for example, onthe intermediate transfer belt 6. In this processing, the detection ofthe mark position (the edge or the center of gravity) of theregistration mark CR is done in such a way that the light travelingafter being reflected by the intermediate transfer belt 6 is detected.The edge detection data are recorded in a RAM or the like, andafterwards, on the basis of the record, the amounts of positionaldeviations for the component colors Y, M, C, and BK are calculated; theexposure timing at the exposure means for the respective colors arecontrolled in such a way as to superpose the toner images with theseamounts of positional deviations reduced to zero.

FIG. 55 is a drawing showing an example of the waveform of a signal of ascratch obtained by the registration sensor 12A or the like. In FIG. 55,the abscissa represents time t, and the ordinate represents the signallevel of a position detection signal S2 obtained by the registrationsensor 12A or the like. The solid line in FIG. 55 shows the waveform ofa signal representing the state of the color image formation surface ofthe intermediate transfer belt 6 before the formation of the colorregistration marks. Lb denotes the base correction level of the positiondetection signal S2, and Lth denotes a threshold voltage.

This waveform can be obtained by the detection of the color imageformation surface by means of the registration sensor 12A or the likethrough one revolution of the intermediate transfer belt 6. Whether ornot the interception of light by the color registration marks does occurin the detection processing is judged by whether or not a signal levellower than the threshold voltage Lth does exist. According to the signalexample shown in FIG. 55, values of the position detection signal S2coming lower than the threshold voltage Lth are detected; this is thecase where the intermediate transfer belt 6 has scratches etc. toobstruct the color registration mark detection processing. It sometimesoccurs that scratches are produced when the intermediate transfer belt 6is rubbed by the drum 1 which is suddenly stopped due to a power failureduring the operation of the image forming apparatus, or when theintermediate transfer belt 6 is taken out and inserted for maintenanceoperation.

Incidentally, according to a tandem-type color image forming apparatusbased on a conventional method, there are problems as described below.

(1) In the color registration mark detection processing, owing to thechange of the intermediate transfer belt 6 with the passage of time, forexample, scratches on the belt surface, although they are differentdepending on the material of the belt, the reflected light quantity bythe color toner layer is raised, which makes it difficult to securesufficiently the signal difference between the toner surface and thesubstrate surface. In particular, if the image density of the colorregistration marks CR fluctuates, that is, if the density becomes lower,or if the density becomes too much higher, the reflected light quantitybecomes higher; therefore, there is a possible risk that the colorregistration marks cannot be detected with a good reproducibility due tothe density fluctuation.

(2) If scratches (permanent scratches) increase on the belt owing to theabove-mentioned change of the intermediate transfer belt 6 with thepassage of time, it occurs frequently the case where the sensors reactto the scratches etc. on the belt, which makes it impossible to detectexactly the mark edge of the color registration marks (hereinafterreferred to also as the mark images). In this connection, it can bethought of a method in which a black colored band is formed on the colorimage formation surface of the intermediate transfer belt 6 beforehand,and afterwards, registration marks of the respective colors are formedon this black surface, but this makes toner consumption larger.

(3) If scratches (permanent scratches) increase on the belt owing to thechange of the intermediate transfer belt (hereinafter referred to alsoas the image transfer system) with the passage of time, in the colorregistration mark detection processing, it occurs frequently the casewhere the sensors react to the scratches etc. on the belt, which makesit impossible to detect exactly the mark edge of the color registrationmarks. In such a case, it is necessary to take trouble to form the colorregistration marks at positions of no belt scratches.

(4) In this connection, for a method of eliminating scratch data fromposition detection data containing belt scratches, it has been thoughtof a method in which registration marks are formed, afterwards, theperiods of time from a reference timing to the line edges of each of theregistration marks are memorized, this processing is repeated pluraltimes, and the detection of all the registration marks is tested; then,only the places where the registration marks overlap one another areextracted. However, as regards this method, in order to eliminate beltscratches etc. which appear unexpectedly, the above-mentioned processinghas to be repeated plural times every occasion, it takes a considerabletime for the calculation of the color deviation amounts of one time.

SUMMARY OF THE INVENTION

Thus, this invention has solved the above-mentioned problems (1) and(2); its first object is to provide an image forming apparatus and animage forming method which makes it possible, even if the condition ofuse of the image transfer system changes with the passage of time, todetect exactly the proper position of mark images, and also makes itpossible to adjust the forming positions of component color images witha good accuracy on the basis of position detection signals of highreliability.

Further, this invention has solved also the above-mentioned problems (3)and (4); its second object is to provide an image forming apparatus andan image forming method which makes it unnecessary, even if thecondition of use of the image transfer system changes with the passageof time, to take trouble to form mark images avoiding belt scratchesetc., and makes it possible, even if a plurality of scratches areproduced on the image transfer system, to extract the positional datanot influenced by the scratches for each of the component colors.

The above-mentioned first object can be accomplished by either one ofthe following aspects (1) to (6) of the present invention.

(1) A first image forming apparatus according to the present inventionis an apparatus for forming a color image on the basis of arbitraryimage information for the color image, comprising an image forming meansequipped with image forming members for forming the color image throughthe superposition of component color images on the basis of thearbitrary image information, a first detection means for detecting thedensity of the color image formed by the image forming means, a seconddetection means for detecting the position of the color image formed bythe image forming means, and a control device for controlling the imageforming means on the basis of output of the first and second detectionmeans, characterized in that the control means controls the seconddetection means so as to detect the density of patch images for thecorrection of color density, and corrects a binarization reference valueto be used for detecting the positions of mark images for theregistration of the component color images on the basis of a detecteddensity signal of the patch images outputted from the second detectionmeans.

By the first image forming apparatus according to the present invention,in the case where a color image is formed by image forming means throughthe superposition of the component color images on the basis ofarbitrary image information, for example, by an image forming unit tomake up the image forming means, a component color image is formed on animage transfer means. The density of this component color image formedon this image transfer means is detected by a first detection means.Further, the position of the component color image formed on the imagetransfer means is detected by a second detection means. Output signalsfrom the first and second detection means are inputted to a controldevice, and on the basis of these output signals, the image transfermeans or the image forming units are controlled.

With this taken as a prior condition, the control device detects thedensity of patch images for the correction of color density by thesecond detection means, and corrects a binarization reference value tobe used for detecting the positions of mark images for the registrationof the component color images on the basis of a detected density signalof the patch image outputted from the second detection means.

Accordingly, it is possible to correct the binarization reference valuefor detecting positions of the registration mark images in such a way asto adapt it to the state of use of the image transfer means or imageforming units in accordance with the condition of use. Owing to this,even if the condition of use of the image transfer means changes withthe passage of time due to the change of the reflected light quantity atthe image transfer means, the reduction of the emission quantity of thesensors, etc., the proper positions of the registration mark images canbe detected exactly; therefore, the forming positions of component colorimages can be adjusted at a high accuracy on the basis of a positiondetection signal of high reliability.

(2) A first image forming method according to the present invention isan image forming method for forming a color image by means of an imageforming system through the superposition of the component color imageson the basis of arbitrary image information, characterized in that inthe case where a color image is formed by the image forming system, thedensity of the color image formed by the image forming system isdetected by means of a first detection system, the position of the colorimage is detected by means of a second detection system, and the imageforming system is controlled on the basis of the output signals from thefirst and second detection systems, patch images for correcting colordensity is previously formed by the image forming system, the density ofthe patch images formed by the image forming system is detected by thesecond detection system, and on the basis of the density detectionsignal of the patch image outputted from the second detection system, abinarization reference value to be used for detecting the positions ofmark images for the registration of component color images is corrected.

By the first image forming method according to the present invention, itis possible to make a correction such that a binarization referencevalue for detecting the positions of mark images is adapted, forexample, to the state of use of the image transfer system or the imageforming system in accordance with the condition of use.

Accordingly, even if the condition of use changes with the passage oftime due to the change of the reflected light quantity at the imagetransfer system or the image forming system, the reduction of theemission quantity of the sensors, etc., the proper positions of markimages can be detected exactly; therefore, the forming positions ofcomponent color images can be adjusted at a high accuracy on the basisof a position detection signal of high reliability. Owing to this,component color images can be superposed exactly in the image formingsystem or in the image transfer system; therefore, a color image can betransferred onto a desired transfer paper sheet at a high accuracy.

(3) A second image forming apparatus according to the present inventionis an apparatus for forming a color image on the basis of arbitraryimage information for a color image, comprising an image forming meansequipped with an image forming member for forming the color imagethrough the superposition of component color images on the basis of thearbitrary image information, a detection means for detecting theposition of the color image formed by the image forming means, and acontrol device for controlling the image forming means on the basis ofthe output of this detection means, characterized in that the controldevice forms beforehand at least reversed mark images which are thereversed ones of mark images for the registration of component colorimages formed by the image forming means, and controls the image formingmeans in such a way as to adjust the forming positions of componentcolor images on the basis of the position detection of mark imagesdefined by the void portions (portions not filled with color tonerparticles) of the reversed mark images formed by the image formingmeans.

By the second image forming apparatus according to the presentinvention, in order that a color image may be formed through thesuperposition of component color images on the basis of arbitrary imageinformation, for example, a color image is formed on an image transfermeans by an image forming unit making up an image forming means. Theposition of the color image formed on this image transfer means isdetected by a detection means. In the control device, it is practicedthat the image transfer means or the image forming unit is controlled onthe basis of the output of the detection means.

With this taken as a prior condition, the control device previouslyforms reversed mark images on the image transfer means, and afterwards,it controls the image forming unit in such a way as to adjust theforming positions of component color images on the basis of thepositions of these mark images defined by the void portions of thereversed mark images formed on the image transfer means.

Accordingly, because the area other than the void portions defining themark images can be covered by the reversed mark images, even ifscratches etc. are produced on the image transfer means due to a changewith the passage of time caused by maintenance operations or wear ofparts, the proper positions of the mark images can be exactly detected.Owing to this, the forming positions of component color images can beadjusted at a high accuracy on the basis of a high-reliability positiondetection signal with no noise signal due to scratches or the likesuperposed.

(4) A second image forming method according to the present invention isan image forming method for forming a color image through thesuperposition of component color images on the basis of arbitrary imageinformation, characterized in that reversed mark image information forreversing mark images for the registration of component color images isprepared beforehand, reversed mark images are formed by an image formingsystem on the basis of this reversed mark image information, theposition of the mark images defined by the void portions of the reversedmark images formed by the image forming system is detected, and theforming positions of component color images are adjusted on the basis ofthe positions of the mark images defined by the void portions.

By the second image forming method according to the present invention,because reversed mark images defined as mark images by the void portionscan be formed on an image transfer system or image forming member with agood reproducibility, scratches etc. which are produced on the imagetransfer system at the time of a maintenance operation or the likeagainst the proper object can be covered by the reversed mark images.

Further, even if scratches etc. are produced on an image transfer systemor image forming member, the proper positions of mark images can bedetected; therefore, the forming positions of component color images canbe adjusted at a high accuracy on the basis of a high-reliabilityposition detection signal with no noise signal due to scratches or thelike superposed. Accordingly, because component color images can besuperposed exactly on the image transfer system, a color image can betransferred onto a desired transfer paper sheet at a high accuracy.

(5) A third image forming apparatus is an apparatus for forming a colorimage on the basis of arbitrary image information for the color image,comprising an image forming means equipped with an image forming bodyfor forming the color image through the superposition of component colorimages on the basis of the arbitrary image information, a detectionmeans for detecting the position of a color image formed by the imageforming means, and a control device for controlling the image formingmeans on the basis of the output of this detection means, characterizedin that the control device forms beforehand, at least, in accordancewith the state of use, mark images or reversed mark images which are thereversed ones of mark images for the registration of component colorimages formed by the image forming means according to a condition of useof the image forming means, and controls the image forming means in sucha way as to adjust the forming positions of component color images onthe basis of the position detection of mark images or those defined bythe void portions of the reversed mark images formed by the imageforming means.

By the third image forming apparatus according to the present invention,in order that a color image may be formed through the superposition ofcomponent color images on the basis of arbitrary image information, forexample, the color image is formed on an image transfer means by animage forming unit making up an image forming means. The position of thecolor image formed on image transfer means is detected by a detectionmeans for detecting a position. In a control device, the image transfermeans or the image forming unit is controlled on the basis of the outputof the detection means.

With this taken as a prior condition, the control device forms markimages or reversed mark images which are the reversed ones of the markimages on the image transfer means in accordance with the state of useof the image transfer means, and controls the image forming unit in sucha way as to adjust the forming positions of component color images onthe basis of the position detection of mark images or those defined bythe void portions of the reversed mark images formed on the imagetransfer means.

Accordingly, for example, in the case where an image transfer means, animage forming member, or the like is used for the first time, or in thecase where an image transfer means has been just replaced with a newone, the forming positions of component color images can be adjusted onthe basis of the position detection of mark images. In the case wherescratches etc. are produced on the image transfer means due to a changewith the passage of time caused by maintenance operations, wear ofparts, etc., scratches etc. can be covered by reversed mark images;therefore, the proper positions of mark images can be exactly detected.

Owing to this, the forming positions of the component color images canbe adjusted at a high accuracy on the basis of a high-reliabilityposition detection signal with no noise signal caused by scratches etc.superposed.

(6) A third image forming method according to the present invention isan image forming method for forming a color image through thesuperposition of component color images by an image forming system onthe basis of arbitrary image information, characterized by comprisingthe steps of previously preparing mark image information for formingmark images for the registration of component color images or reversedmark image information for forming reversed mark images which are thereversed ones of the mark images, afterwards, detecting the surfacestate of the color image formation area of the image forming system,forming mark images based on the mark image information or reversed markimages based on the reversed mark image information by an image transfersystem in accordance with the surface state of the color image formationarea, and adjusting the forming positions of component color images onthe basis of the position detection of mark images or those defined bythe void portions of the reversed mark images.

By the third image forming method according to the present invention,for example, in the case where an image transfer means, an image formingmember, or the like is used for the first time, or in the case where theimage transfer means has been just replaced with a new one, the formingpositions of the component color images can be adjusted on the basis ofposition detection of the mark images. In the case where scratches etc.are produced on the image transfer means due to a change with thepassage of time caused by maintenance operations, wear of parts, etc.,scratches etc. can be covered by reversed mark images; therefore, theproper positions of mark images can be exactly detected.

Accordingly, the forming positions of the component color images can beadjusted at a high accuracy on the basis of a high-reliability positiondetection signal with no noise signal caused by scratches etc.superposed. Owing to this, because component color images can be exactlysuperposed on an image transfer means, a color image can be transferredonto a desired transfer paper sheet at a high accuracy.

The above-mentioned second object can be accomplished by either one ofthe following aspects (7) and (8) of the present invention.

(7) A fourth image forming apparatus according to the present inventionis an apparatus for forming a color image through the superposition ofcomponent color images on the basis of arbitrary image information,comprising an image transfer means, image forming units for formingplural mark images for each of the component colors for the registrationof component color images on the image transfer means, detection meansfor detecting the positions of the mark images formed by the imageforming units, and a control device for controlling the image transfermeans and/or image forming units on the basis of the output of thedetection means, characterized in that the control device obtainsposition data representing the forming position of each of the markimages with respect to a reference value arbitrarily determinedconcerning the mark images for the registration of the component colorimages formed on the image transfer means, divides the position data ofthe mark images of each of the component colors into groups for theirrespective unit reference ranges, concerning the position data in therespective unit reference ranges divided in the above, makes anoperation processing for converting the position data divided for eachunit reference range into those based on respective reference valuesrepresenting the front edge of the unit reference ranges concerned,extracts the position data representing ranges common to the mutuallyoverlapping ranges each of which is represented by a couple of positiondata corresponding to the rising and falling edges of a passage timingpulse subjected to the operation processing as read position data of themark image of the color concerned for the registration of componentcolor images, and calculates the amounts of positional deviations of thecomponent color images from one another on the basis of the positiondata extracted.

By the fourth image forming apparatus according to the presentinvention, in order that a color image may be formed through thesuperposition of component color images on the basis of arbitrary imageinformation, plural mark images of the respective colors for theregistration of component color images are formed by means of imageforming units on the image transfer means. The positions of the markimages formed on the image transfer means are detected by the detectionmeans. The control device controls the image transfer means and/or theimage forming units on the basis of the output of the detection means.

With this taken as a prior condition, the control device obtainsposition data representing the forming position of each of the markimages with respect to an arbitrarily determined reference valueconcerning the mark images for the registration of component colorimages formed on the image transfer means, divides the position data ofthe mark images of the respective colors into groups for theirrespective unit reference ranges, concerning the position data in therespective unit reference ranges divided in the above, makes anoperation processing for converting them into those based on respectivereference values representing the front edge of the unit referenceranges concerned, extracts the position data representing ranges commonto the mutually overlapping ranges each of which is represented by acouple of position data corresponding to the rising and falling edges ofa passage timing pulse subjected to the operation processing as readpositional data of the mark image of the color concerned for theregistration of component color images, and calculates the amounts ofpositional deviation of the component color images from one another onthe basis of the position data extracted in the above.

Accordingly, it is possible to eliminate the position data representinga range not overlapped by any other range which is represented by acouple of position data corresponding to the rising and falling edges ofa passage timing pulse as position data concerning scratches. Owing tothis, even if position data containing those concerning scratches causedby the use of the image transfer means during the passage of a period oftime, position data which are not influenced by scratches can beextracted for each of the component colors. For this reason, it isunnecessary to take trouble to avoid scratches for the formation of markimages, and further, even if a plurality of scratches are produced onthe image transfer means, position data which are not influenced byscratches can be extracted for each of the component colors. Owing tothis, the forming positions of component color images can be adjusted ata high accuracy on the basis of high-reliability position data notinfluenced by a noise signal caused by scratches etc.

Furthermore, for position data of a plurality of mark images formed onthe image transfer means, the removal processing of scratches has onlyto be practiced one time; therefore, it is unnecessary to practice themark image formation sequence plural times, which makes it possible,compared to a conventional method, to shorten the calculation time forthe amounts of positional deviations.

(8) A fourth image forming method according to the present invention isan image forming method for forming a color image through thesuperposition of component color images on the basis of arbitrary imageinformation, characterized by comprising the steps of forming pluralmark images of the respective colors for the registration of componentcolor images on an image transfer system with an image formation regionof one mark image defined as a unit reference range, while previouslydetermining a reference value concerning each of the mark images for theregistration of component color images formed on the image transfersystem, obtaining, concerning the mark images for the registration ofcomponent color images formed on the image transfer system, positiondata representing the respective forming positions of the mark imageswith respect to the reference values, dividing the position data of themark images of the respective colors obtained in the above for each unitreference range, concerning the position data divided for each unitreference range, carrying out an operation processing for convertingthem into those based on respective reference values representing thefront edge of the unit reference ranges concerned, extracting theposition data representing ranges common to the mutually overlappingranges each of which is represented by a couple of position datacorresponding to the rising and falling edges of a passage timing pulsesubjected to the above-mentioned operation processing as position dataof the respective colors for the registration of component color images,and calculating the amounts of deviations of the component color imagesfrom one another on the basis of the position data extracted in theabove.

By the fourth image forming method according to the present invention,in order that a color image may be formed through the superposition ofcomponent color images on the basis of arbitrary image information, theposition data representing a range not overlapped by any other rangeeach of which is represented by a couple of position data correspondingto the rising and falling edges of a passage timing pulse can beeliminated as position data concerning scratches. Accordingly, even inthe case where position data containing those concerning scratchescaused by the use of the image transfer system during the passage of aperiod of time are obtained, it is possible to extract position data notinfluenced by scratches for each of the colors.

For this reason, it is unnecessary to take trouble to avoid scratchesfor the formation of mark images, and further, even if a plurality ofscratches are produced on the image transfer system, positional datawhich are not influenced by scratches can be extracted for each of thecolors. Owing to this, the position of color image formation can beadjusted at a high accuracy on the basis of high-reliability positiondata not influenced by a noise signal caused by scratches etc.

Furthermore, for position data of a plurality of series of mark imagesformed on the image transfer system, the removal processing of scratcheshas only to be practiced one time; therefore, it is unnecessary topractice the mark image formation sequence plural times, which makes itpossible, compared to a conventional method, to shorten the calculationtime for the amounts of positional deviations. Owing to this, a colorimage can be transferred onto a desired transfer paper sheet at a highaccuracy, because the component color images can be exactly superposedon the image transfer system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual drawing showing an example of the structure of acolor image forming apparatus 100 as the embodiment 1 and the embodiment7 of this invention;

FIG. 2 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 100 as the embodiment 1 and the embodiment 7 of thisinvention;

FIG. 3 is a block diagram showing an example of the internal structureof the positional deviation control system of a control device 15;

FIG. 4 is a conceptual drawing showing an example of the structure of aimage writing section 3Y and its correction means 5Y for the color Y;

FIG. 5 is a perspective view showing an example of the arrangement of atoner image density sensor 11 and registration sensors 12A and 12B inthe embodiment 1;

FIG. 6(A)-1 to FIG. 6(A)-4 are drawings showing patch marks Pm and FIG.6B is a drawing showing an example of the waveform representing anexample of the density detection of the patch marks by the registrationsensor 12A or the like;

FIG. 7(A) and FIG. 7(B) are drawings showing examples of the waveform ofa density detection signal S2′ by the registration sensor 12A or thelike;

FIG. 8 is a waveform drawing showing an example of threshold settingbased on the density detection of the patch marks Pm;

FIG. 9 is a perspective view showing an example of the detection ofcolor registration marks CR by the registration sensors 12A and 12B inthe embodiment 1 and embodiment 7;

FIG. 10(A) and FIG. 10(B) showing an example of binarization of aposition detection signal S2 by the registration sensor 12A or the like;

FIG. 11 is a flow chart showing an example of the operation of the colorimage forming apparatus 100 in the embodiment 1;

FIG. 12 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 200 as the embodiment 2 of this invention;

FIG. 13(A) and FIG. 13(B) are image drawings showing the examples of thestructure of a mark image and a reversed mark image;

FIG. 14 is a conceptual drawing showing an example of the formation ofreversed color registration marks RCR for the colors BK, C, M, and Y;

FIG. 15 is a flow chart showing an example of the operation of the colorimage forming apparatus 200;

FIG. 16 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 300 as the embodiment 3 of this invention;

FIG. 17 is a block diagram showing an example of the internal structureof the positional deviation control system and the image formationcontrol system of the control device 15;

FIG. 18(A) and FIG. 18(B) are circuit diagrams showing an example of thestructure of a new belt detection circuit 90;

FIG. 19(A) to FIG. 19(C) are conceptual drawings showing examples of anoperation screen P1 at the time of image formation in a display device29;

FIG. 20 is a conceptual drawing showing an example of the formation ofnon-reversed color registration marks CR and reversed color registrationmarks RCR on an image transfer member 6;

FIG. 21 is a perspective view showing another example of the arrangementof the registration sensor 12A, etc.;

FIG. 22 is a waveform drawing showing an example of a signal in the caseof a base level correction by the registration sensor 12A or the like;

FIG. 23 is a flow chart showing an example of the operation (at the timeof detecting the presence or absence of a scratch) of the color imageforming apparatus 300 as the first example of this invention;

FIG. 24 is a flow chart showing an example of the setting of apositional deviation adjustment mode in the color image formingapparatus 300;

FIG. 25 is a flow chart showing another example of the operation (at thetime of detecting the presence or absence of a scratch) of the colorimage forming apparatus 300;

FIG. 26 is a flow chart showing an example of the operation of the colorimage forming apparatus 300 as the second example of this invention;

FIG. 27 is a flow chart showing an example of the operation of the colorimage forming apparatus 300 as the third example of this invention;

FIG. 28 is a conceptual drawing showing an example of the structure of acolor image forming apparatus 400 as another example of the embodimentof this invention;

FIG. 29 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 400 as the embodiment 4 of this invention;

FIG. 30 is a conceptual drawing showing an example of the structure of awriting section for the Y-color 3Y and its correction means 5Y;

FIG. 31 is a perspective view showing an example of the arrangement ofthe toner image density sensor 11 and the registration sensors 12A and12B;

FIG. 32 is a perspective view showing an example of the detection ofcolor registration marks CR by the registration sensors 12A and 12B;

FIG. 33 is a flow chart showing an example of the operation of the colorimage forming apparatus 400;

FIG. 34 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 500 as the embodiment 5 of this invention;

FIG. 35 is a conceptual drawing showing an example of the formation ofreversed color registration marks RCR for the colors BK, C, M, and T;

FIG. 36 is a flow chart showing an example of the operation of the colorimage forming apparatus 500;

FIG. 37 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 600 as the embodiment 6 of this invention;

FIG. 38 is a conceptual drawing showing an example of the formation ofnon-reversed color registration marks CR and reversed color registrationmarks RCR on a photoreceptor belt 60;

FIG. 39 is a perspective view showing another example of the arrangementof the registration sensors 12A etc.;

FIG. 40 is a flow chart showing an example of operation (at the time ofdetecting the presence or absence of a scratch) of the color imageforming apparatus 600 as the first example of this invention;

FIG. 41 is a flow chart showing an example of the setting of apositional deviation adjustment mode in the color image formingapparatus 600;

FIG. 42 is a flow chart showing another example of the operation (at thetime of detecting the presence or absence of a scratch) of the colorimage forming apparatus 600;

FIG. 43 is a flow chart showing an example of the operation of the colorimage forming apparatus 600 as the second example of this invention;

FIG. 44 is a flow chart showing an example of the operation of the colorimage forming apparatus 600 as the third example of this invention;

FIG. 45(A) and FIG. 45(B) are drawings showing an example of theprinting of a color registration mark CR and an example of the waveformof a passage timing pulse signal SP based on this in the embodiment 7 ofthis invention;

FIG. 46(A) to FIG. 46(D) are waveform drawings showing an example of theoutput of passage time data DT based on a passage timing pulse signal;

FIG. 47(A) to FIG. 47(C) are drawings showing an example of the passagetime data DT and the position data DP concerning the forming positionsof the color registration marks of the first color to the fourth color(Y, M, C, and BK);

FIG. 48(A) and FIG. 48(B) are a drawing showing an example of theformation of color registration marks of the first color to the fourthcolor and a conceptual drawing showing an example of the setting of unitbasic ranges Pr in color registration marks CR of the first colorrespectively;

FIG. 49 is a drawing showing an example of a belt scratch 9 happening tocome in the registration mark MARK1;

FIG. 50(A) to FIG. 50(E) are drawings showing an example of theoperation for eliminating image data produced by the belt scratch 9shown in FIG. 49;

FIG. 51 is a conceptual drawing showing an example of the correction ofa positional deviation by means of the CPU 55;

FIG. 52 is a flow chart showing an example of the operation of the colorimage forming apparatus 100 in the embodiment 7;

FIG. 53 is a flow chart showing an example of the acquisition ofposition data DP based on passage time data DT;

FIG. 54 is a drawing showing an example of the detection of colorregistration marks according to a prior art; and

FIG. 55 is a waveform drawing showing an example of the signal of ascratch obtained by the registration sensor 12A or the like.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT (1) Embodiment 1

FIG. 1 is a conceptual drawing showing an example of the structure of acolor image forming apparatus 100 as an example of the embodiment ofthis invention.

In this example of the embodiment, it is put into practice that there isprovided a control device for controlling an image transfer means or aimage forming unit on the basis of output signals of a density detectionsystem and a position detection system for a color image, wherein thedensity of patch images for color density correction is detected by theposition detection system for a color image, and a binarizationreference value (also called a control reference value) for thedetection of the positions of mark images is corrected on the basis ofthe density detection signal of the patch images outputted from saidposition detection system. Thus, even if the condition of use of theimage transfer means has changed with the passage of time due to thechange of reflected light quantity at the image transfer means, thereduction of the emission quantity of sensors, etc., the properpositions of the mark images can be exactly detected, and on the basisof a high-reliability position detection signal, the forming positionsof component color images can be adjusted at a high accuracy.

The color image forming apparatus 100 shown in FIG. 1 has an example ofthe structure of the first to third image forming apparatus according tothe aspects of the invention (1) to (3), and is an apparatus for forminga color image on an image transfer system through the superposition ofcomponent color images on the basis of arbitrary image information.

In FIG. 1, the color image forming apparatus 100 is made up of an imageforming apparatus mainframe 101 and an image reading apparatus 102. Onthe image forming apparatus mainframe 101, the image reading apparatus102 made up of an automatic document feeder 201 and a document imagescanning exposure device 202 is mounted. A document sheet d placed onthe document table of the automatic document feeder 201 is conveyed by aconveyance means, and by means of the optical system of the documentimage scanning exposure device 202, an image on one or both sides of thedocument is subjected to scanning exposure, and is read by a line imagesensor CCD.

The analog signals obtained by the reading of the line image sensor CCDare subjected to an analog processing, A/D conversion, a shadingcorrection processing, an image compression processing, etc. in an imageprocessing section (not shown in the drawing), to become imageinformation. After that, the image information is transmitted to imagewriting sections (exposure means) 3Y, 3M, 3C, and 3K, which make up therespective image forming units.

The automatic document feeder 201 is equipped with an automaticdouble-sided document conveyance means. This automatic document feeder201 reads the content of a multi-page document d fed from on thedocument table by a single continuous run, and the content of thedocument is accumulated in a storage means (an electronic RDH function).This electronic RDH function is conveniently used when the content of amulti-page document is copied by the copying function, or when amulti-page document d is transmitted by the facsimile function, forexample.

The image forming apparatus mainframe 101 is what is called atandem-type color image forming apparatus, and is composed of aplurality of image forming units (image forming system) 10Y, 10M, 10C,and 10K, an endless intermediate transfer belt 6 as an intermediatetransfer member which is an example of the image transfer means (theimage transfer system), a paper feed-conveyance means containing are-feed mechanism (an ADU mechanism), and a fixing device 17 for fixinga toner image.

The image forming unit 10Y for forming an image of the color yellow (Y)comprises a photoreceptor drum 1Y as an image forming member, and acharging means 2Y, an exposure means 3Y, a developing device 4Y and acleaning means 8Y for the image forming member for the color Y arrangedat the circumference of the photoreceptor drum 1Y. The image formingunit 10M for forming an image of the color magenta (M) comprises aphotoreceptor drum 1M as an image forming member, and a charging means2M, an exposure means 3M, a developing device 4M, and a cleaning means8M for the image forming member for the color M.

The image forming unit 10C for forming an image of the color cyan (C)comprises a photoreceptor drum 1C as an image forming member, and acharging means 2C, an exposure means 3C, a developing device 4C, and acleaning means 8C for the image forming member for the color C. Theimage forming unit 10K for forming an image of the color black (BK)comprises a photoreceptor drum 1K as an image forming member, and acharging means 2K, an exposure means 3K, a developing device 4K, and acleaning means 8K for the image forming member for the color BK.

The combinations of the charging means 2Y and the exposure means 3Y, thecharging means 2M and the exposure means 3M, the charging means 2C andthe exposure means 3C, and the charging means 2K and the exposure means3K make up latent image forming means respectively. As regards thedevelopment by means of the developing devices 4Y, 4M, 4C, or 4K, it ispracticed a reverse development process with a developing bias voltagecomposed of a direct-current voltage having the polarity the same asthat of the toners used (negative polarity in this example of theembodiment) and an alternate-current voltage superposed applied. Theintermediate transfer belt 6 is entrained about a plurality of rollersand is supported in a way to be able to revolve.

The outline of an image forming process will be explained in thefollowing. Images of the respective colors formed by the image formingunits 10Y, 10M, 10C, and 19K are transferred successively onto therevolving intermediate transfer belt 6 by the primary transfer rollers7Y, 7M, 7C, and 7K having a primary transfer bias voltage (not shown inthe drawing) of the polarity reverse to the toners used (positivepolarity in this example of the embodiment) applied respectively(primary transfer), and a synthesized color image (color toner image) isformed. The color image is transferred from the intermediate transferbelt 6 to a paper sheet P.

A paper sheet P contained in a paper feed cassette 20A, 20B, or 20C isfed by a conveying-out roller 21 and feed roller 22A which are providedin each of the paper feed cassette 20A, 20B, and 20C, and is conveyedthrough conveyance rollers 22B, 22C, and 22D, a registration roller 23,etc. to a secondary transfer roller 7A; thus, on one side (front side)of the paper sheet P, the color image is transferred (secondarytransfer).

The paper sheet P, having a color image transferred on it, is subjectedto the fixing process by the fixing device 17, and is gripped by a pairof ejection rollers 17, to be placed on an output tray 25 outside themachine. The residual toner particles remaining on the circumferentialsurface of the photoreceptor drums 1Y, 1M, 1C, and 1K are removed by theimage forming member cleaning means 8Y, 8M, 8C, and 8K, and the nextimage formation cycle will start.

When another image is to be formed on the other side, the paper sheet P,having been discharged out of the fixing device 17, diverges from thesheet ejection path by a bifurcating means 26, passes through acirculating path 27A provided downward, is inverted upside down by aninversion conveyance path, which is the re-feed mechanism (ADUmechanism), passes the re-feed conveyance section 27C, and comes back tothe former path at the downstream side of the conveyance roller 22D.

The paper sheet P, having be subjected to the inversion conveyance,passes through the registration roller 23, and is again conveyed to thesecondary transfer roller 7A, by which another color image (a colortoner image) is transferred on the other side (rear side) of the papersheet P. The paper sheet P, having color images transferred on both thesides, are gripped by the ejection rollers to be placed on the outputtray 25 outside the machine.

On the other hand, after the color image is transferred onto a papersheet P by the secondary transfer roller 7A, the intermediate transferbelt 6, having the paper sheet P detached off from it by the curvature,is cleaned through removing the residual toner particles by the cleaningmeans for an intermediate transfer belt 8A. In the case of such imageformations, it is desirable that for the paper sheet P, a thin papersheet of the order 52.3 g/m² to 63.9 g/m², a normal paper sheet of theorder 64.0 g/m² to 81.4 g/m², a thick paper sheet of the order 83.0 g/m²to 130.0 g/m², or a super thick paper sheet of the order 150.0 g/m² isused, the process speed is made to fall within a range of 80 mm/s to 350mm/s, and for the environment condition, the temperature is set to fallwithin a range of 5° C. to 35° C., and the humidity is set to fallwithin a range of 15% to 85%. As regards the thickness of the papersheet, one having a thickness falling within a range of 0.05 mm to 0.15mm is used.

At the left of the intermediate transfer belt 6 in the upstream side ofthe above-mentioned cleaning means 8A, there is provided a sensor fordetecting toner image density as an example of the first detection means(hereinafter referred to as a toner image density sensor 11 simply),which detects the density of a toner image formed on the intermediatetransfer belt 6, to generate a density detection signal S1.

Located by this toner image density sensor, there is provided a sensorfor detecting the positional deviation of a toner image as an example ofthe second detection means (hereinafter referred to as a registrationsensor 12 simply), which detects the position of a mark image formed onthe intermediate transfer belt 6 (hereinafter referred to as a colorregistration mark CR) to generate a position detection signal S2. In theimage forming apparatus mainframe 101, there is provided a controldevice 15, which practices color registration mark detection processingon the basis of a density detection signal S1 and a position detectionsignal S2.

The color registration mark processing means the processing such thatcolor registration marks CR for the registration of component colorimages are formed on the intermediate transfer belt 6, and the positions(the edge, the gravity center, or the like) of these color registrationmarks formed on the intermediate transfer belt 6 are detected by theregistration sensor 12. This processing is practiced for the purpose ofadjusting the forming positions of component color images on the basisof the positions of the color registration marks CR. In this example,even if the condition of use of the intermediate transfer belt 6 changeswith the passage of time, the proper positions of the color registrationmarks CR can be made to be detected exactly, and it is made possible toadjust the forming positions of component color images at a highaccuracy on the basis of a high-reliability position detection signalS2.

FIG. 2 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of the color imageforming apparatus 100 as the embodiment 1 of the present invention. FIG.2 is an enlarged drawing of the intermediate transfer belt 6 and theimage forming units 10Y, 10M, 10C, and 10K of the color image formingapparatus 100 of FIG. 1 shown as an image transfer system I and an imageforming system II, respectively. In FIG. 2, the color image formingapparatus 100 has the control device 15. The toner image density sensor11 is connected to the control device 15, detects the density of a tonerimage formed on the intermediate transfer belt 6, and outputs a densitydetection signal S1 to the control device 15.

To the control device 15, in addition to the toner image density sensor11, the registration sensors 12 are connected; each of them detects theposition of a toner image formed on the intermediate transfer belt 6,and outputs a position detection signal S2 to the control device 15. Thecontrol device 15 is given a role to control the image forming units10Y, 10M, 10C, and 10K on the basis of a density detection signal S1obtained from the toner image density sensor 11 and position detectionsignals S2 obtained from the registration sensors 12. In this example,corrections concerning the image forming units 10Y, 10M, 10C, and 10K,etc., the adjustment of the write start position in the main scanningand sub-scanning and the position adjustment in a writing area (skewadjustment), the correction of the main scanning write clock signal(lateral magnification adjustment/partial lateral magnificationadjustment), etc. are made (refer to FIG. 3).

Although it depends on the contents of the control, it is alsoappropriate to adopt a method such that one or all of the three imageforming units 10Y, 10M, and 10C are controlled with the image formingunit 10K taken as the reference; this can reduce the burden of thecontrol device 15. Of course, it is also possible to incorporate theintermediate transfer belt 6 into the object of the control. In thatcase, it is appropriate to provide a zigzag run correction mechanism(not shown in the drawing) and correct a zigzag run of the intermediatetransfer belt 6 for the adjustment of color deviations.

To the control device 15, the image forming units 10Y, 10M, 10C, and 10Kare connected; the operation is as follows: By the image forming unit10Y, a toner image of the color Y is formed on the intermediate transferbelt 6 on the basis of image information for the Y-color Dy, which is acomponent of arbitrary image information Din, by the image forming unit10M, a toner image of the color M is formed on the intermediate transferbelt 6 on the basis of image information for the M-color Dm, by theimage forming unit 10C, a toner image of the color C is formed on theintermediate transfer belt 6 on the basis of image information for theC-color Dc, and by the image forming unit 10K, a toner image of thecolor BK is formed on the intermediate transfer belt 6 on the basis ofimage information for the BK-color Dk.

In this example, a correction means 5Y is attached to the image writingsection (exposure means) for the Y-color 3Y, and operates to adjust theforming position of an image of the color Y on the basis of a writingposition correction signal for the Y-color Sy from the control device15. In the same way, a correction means 5M is attached to the imagewriting section for the M-color 3M, and operates to adjust the formingposition of an image of the color M on the basis of a writing positioncorrection signal for the M-color Sm from the control device 15.

A correction means 5C is attached to the image writing section for theC-color 3C, and operates to adjust the forming position of an image ofthe color C on the basis of a write position correction signal for theC-color Sc from the control device 15. A correction means 5K is attachedto the image writing section for the BK-color 3K, and operates to adjustthe forming position of an image of the color BK on the basis of a writeposition correction signal for the BK-color Sk from the control device15. In this example, as regards the calculation of the amounts of colordeviations, color registration marks CR for the color BK are taken asthe reference. This is done for the purpose of adjusting the writingpositions of component color images of the colors Y, M, and C in such away as to make them agree with the position of a component image of thecolor BK.

For example, as regards the writing position adjustment for the color Y,the writing position of the color registration mark CR of the color BKand the writing position of the color registration mark CR of the colorY are detected, and the correction value is calculated from the amountof deviation to be obtained by the conversion operation of the writingposition of the color registration mark CR for the color Y into thewriting position of the color registration mark CR of the color BK. Inthe same way, also as regards the writing position adjustment for thecolor M or C, the amount of the deviation of the writing position of thecolor registration mark of the color M or C from the writing position ofthe color registration mark of the color BK is detected, and from thisamount of deviation, each correction value is calculated. After that,the image forming units 10Y, 10M, and 10C for the colors Y, M, and Cexcept the image forming unit 10K for the color BK are subjected to theadjustment.

For this purpose, in the image forming unit 10K for the color BK, by theoutput of toner images of the color BK only onto the intermediatetransfer belt 6, a normal writing position adjustment in the mainscanning and sub-scanning and a lateral magnification adjustment, apartial lateral magnification adjustment, a skew adjustment, etc. in theimage writing section 3K are made. This is done for the purpose oftaking the color BK as the reference in the adjustment. After that, itis done to move to the color registration adjustment using a method ofthis invention, in which the registration adjustment for making thewriting positions of images of the colors Y, M, and C agree with thewriting position of an image of the color BK is practiced.

Further, by the control device 15, the image forming units 10Y, 10M, and10C are controlled to form patch marks for color density correction asan example of the aforesaid patch images on the intermediate transferbelt 6. The density of the patch marks formed on the intermediatetransfer belt 6 is detected by the registration sensor 12. After that,the densities of color registration marks CR for the registration ofcomponent color images are adjusted on the basis of the densities of thepatch marks, and the image forming units 10Y, 10M, 10C, and 10K arecontrolled to form color registration marks CR having their densityadjusted in the above-mentioned way on the intermediate transfer belt 6by the control device 15.

The positions of color registration marks formed on the intermediatetransfer belt 6 are detected by the registration sensors 12. The controldevice 15 becomes able to correct in real time the threshold voltage Lthin the detection of the positions of color registration marks CR on thebasis of a density detection signal S2′ of patch marks outputted fromthe registration sensor 12. The image forming units 10Y, 10M, 10C, and10K are subjected to a control to adjust the forming positions ofrespective component color images on the basis of the above-mentionedpositions of color registration marks CR.

In this example, the minimum value of a density detection signal S2′ inthe non-formation area of the patch marks and the maximum value of it inthe formation area of said patch marks outputted from the registrationsensor 12 are detected, and the average value is calculated on the basisof these minimum value and maximum value.

That is, in the case where the output value due to the reflection lightfrom a non-formation area of the patch marks on the intermediatetransfer belt (base) 6 and the output value due to the reflection lightfrom a formation area of the patch marks are close to each other, thecondition for determining the threshold voltage of binarization becomesworst.

For that reason, it is put into practice that, using values of the worstcondition of it, the average value (the median) is derived, and thethreshold voltage of binarization is determined. In this example, it isput into practice that the above-mentioned average value is taken as thebinarization reference value for the detection by the registrationsensors 12, and on the basis of this binarization reference value, thepassage timing of a color registration mark is detected. It is alsoappropriate to make the binarization reference value for the detectionby the registration sensors 12 a value in the neighborhood of theaverage value.

Of course, the process is not limited to this, and also it isappropriate to practice the following processing in one and the samesequence by the control device 15. That is, patch marks for thecorrection of color density are formed on the intermediate transfer belt6, the density of the patch marks formed on the intermediate transferbelt 6 are detected by the toner image density sensor 11, and a densitycorrection processing for adjusting the density of mark images ispracticed, while the density of the patch marks formed on theintermediate transfer belt 6 is detected successively by theregistration sensor 12. By the subsequent detection of the patch marksformed at the time of detecting the patch marks by the registrationsensor 12, the density of a color registration mark CR which makes itpossible to secure the most reliable signal level can be determined.

After that, it is done that the density of color registration marks forthe registration of component color images is adjusted on the basis ofthe density of the patch marks, color registration marks CR having theirdensity adjusted are formed on the intermediate transfer belt 6, thepositions of the color registration marks formed on the intermediatetransfer belt 6 are detected by the registration sensor 12, and aregistration adjustment processing for adjusting the forming positionsof component color images on the basis of the positions of the colorregistration marks CR.

FIG. 3 is a block drawing showing an example of the internal structureof the positional deviation control system of the control device 15. Thecontrol device 15 shown in FIG. 3 comprises an oscillator 51, afrequency divider 52, a polygonal mirror driving circuit 53, a countercircuit 54, a CPU (central processing unit) 55, a latch circuit 56, aRAM 57, a digital/analog (D/A) converter 58, a comparator forbinarization 59, an index delay circuit 510, a VV generating circuit511, an HV generating circuit 512, a skew correction circuit 513, ananalog/digital (A/D) converter 514, a mask generating circuit 515, etc.

The oscillator 51 generates a clock signal CK of the basic frequency.The frequency divider 52 is connected to the oscillator 51, and dividesthe frequency of a clock signal CK to generate a system clock signal SCKof a specified frequency.

The polygonal mirror driving circuit 53 and the counter circuit 54 areconnected to the frequency divider 52. In the polygonal mirror drivingcircuit 53, on the basis of a rotation phase setting signal Sr from theCPU 55, a polygonal mirror drive clock signal for the color Y(hereinafter referred to as a Y polygon CLK), a polygonal mirror driveclock signal for the color M (hereinafter referred to as an M polygonCLK), a polygonal mirror drive clock signal for the color C (hereinafterreferred to as a C polygon CLK), and a polygonal mirror drive clocksignal for the color BK (hereinafter referred to as a BK polygon CLK)are generated from a system clock signal SCK. A Y polygon CLK isoutputted to the image writing section 3Y, an M polygon CLK is outputtedto the image writing section 3M, a C polygon CLK is outputted to theimage writing section 3C, and a BK polygon CLK is outputted to the imagewriting section 3K.

In the counter circuit 54, a system clock signal SCK is counted togenerate a latch signal SL with an image top signal (hereinafterreferred to as a VTOP signal) received from the CPU 55 as a resetsignal. A VTOP signal becomes the reference in detecting the writingposition for the color BK. A latch signal SL becomes a signal showingthe writing position for the color Y, M, or C. For example, the writingposition for the color BK is recognized by counting a system clocksignal SCK with the rise time of the VTOP signal taken as the reference.

The latch circuit 56 is connected to the counter circuit 54, andoperates to latch a latch signal SL on the basis of a passage timingpulse signal Sp after masking. The RAM 57 is connected to the latchcircuit 56, which operates to load the RAM 57. The RAM 57 is connectedto the CPU through a data bus 16.

On the other hand, the registration sensors 12 shown in FIG. 2 areconnected to the comparator 59. Further, the D/A converter 58 isconnected to the comparator 59. The D/A converter 58 makes adigital/analog conversion of a threshold setting data Dth from the CPU55 to output a threshold voltage (binarization reference value) Lth. Inthe comparator 59, a position detection signal S2 from the registrationsensor 12 (an analog signal) is binarized on the basis of the thresholdvoltage Lth. The position detection signal after binarization becomes apassage timing pulse signal Sp. In this example, the most suitablethreshold voltage Lth is obtained by the correction using Dth calculatedby the CPU on the basis of an analog density detection signal S2′ fromthe registration sensor 12 or a density detection signal S1 (an analogsignal) in accordance with the condition of use.

A mask generating circuit 515 is connected to the comparator 59, andoperates to mask the portion of a passage timing pulse signal Sp exceptmark images. The latch circuit is connected to the mask generatingcircuit 515, and operates to control a latch signal SL on the basis of apassage timing pulse signal Sp whose portion other than mark images hasbeen masked.

Further, the registration sensor 12 is connected to the A/D converter514, where a density detection signal S2′ obtained through the detectionof patch marks by said registration sensor 12 is subjected toanalog/digital conversion. Density detection data D1′ after A/Dconversion are outputted to the CPU 55, where a threshold voltagesetting data Dth is determined on the basis of density detection dataD1′. By the use of this threshold voltage setting data Dth, it becomespossible to correct in real time a threshold voltage Lth for detectingthe positions of color registration marks CR.

The above-mentioned density detection sensor 11 is connected to an A/Dconverter 517, where a density detection signal S1 is subjected toanalog/digital conversion. The density detection signal D1 after A/Dconversion is outputted to the CPU 55.

An index delay circuit 510 (hereinafter referred to also as a lateralmagnification correction section) is connected to the CPU 55, andoperates to delay INDEX (clock) signals for the colors Y, M, C, and BKsupplied from an upper-rank control system on the basis of delay controldata D10, to output delayed INDEX signals (delayed Y INDEX, delayed MINDEX, delayed C INDEX, delayed K INDEX) for the colors Y, M, C, and BKto the image forming system.

A VV generation circuit (hereinafter referred to also as a sub-scanningcorrection section) 511 is connected to the CPU 55, and operates togenerate position correction signals for the adjustment of sub-scanningfor the colors Y, M, C, and BK, Sy (YVV), Sm (MW), Sc (CVV), and Sk(KVV) on the basis of VV generation control data D11 for correcting thewriting position in the vertical direction, to output these signals, Sy,Sm, Sc, and Sk to the image forming system II.

An HV generating circuit (hereinafter referred to also as a mainscanning correction section) 512 is connected to the CPU 55, andoperates to generate position correction signals for the adjustment ofmain scanning for the colors Y, M, C, and BK, YHV, MHV, CHV, and KHV onthe basis of HV generation control data D12 for correcting the writingposition in the main scanning direction, to output these signals, YHV,MHV, CHV, and KHV to the image forming system. This makes it possible toadjust writing positions.

A skew correction circuit (hereinafter referred to also as a skewcorrection section) 513 is connected to the CPU 55, and operates togenerate a skew correction signal S13 for the main scanning adjustmentfor the colors Y, M, C, and BK on the basis of skew correction data D13for correcting an image inclination, to output this signal S13 to theimage forming system II. A plurality of motors are connected to the skewcorrection circuit 513, and are controlled on the basis of skewcorrection signals S13.

FIG. 4 is a conceptual drawing showing an example of the structure ofthe image writing section for the Y-color 3Y and its correction means5Y. The image writing section for the Y-color 3Y comprises asemiconductor laser light source 31, optical systems 32 and 33, apolygonal mirror 34, a polygonal mirror motor 35, and an fθ lens 36. Inthe semiconductor laser light source 31, a laser beam is generated onthe basis of image information for the Y-color Dy. A laser beam emittedfrom the semiconductor laser light source 31 has its beam shapecorrected to a specified one by the optical systems.

This light beam is deflected in the main scanning direction by thepolygonal mirror 34. The polygonal mirror 34 is rotated by the polygonalmirror motor 35 on the basis of a Y polygon CLK from the control device15. A light beam deflected by the polygonal mirror 34 is converged onthe photoreceptor drum 1Y by the fθ lens 36.

In this image writing section 3 y, there is provided the correctionmeans 5Y. The correction means 5Y comprises a lens holding mechanism 41,an fθ lens adjusting mechanism 42, etc. An fθ lens 36 is attached to thelens holding mechanism 41, which is movably attached to the fθ lensadjusting mechanism 42. By the fθ lens adjusting mechanism 42, the lensholding mechanism is moved to rotate the fθ lens about its optical axisperpendicular to the image formation surface on the basis of a positioncorrection signal Sy (YVV) for adjustment.

This mechanism 42 is embodied by the use of an actuator (a piezoelectricelement), or by the control of the pitch of a fully threaded bolt. Theabove-mentioned mechanism is provided for the purpose of the adjustmentof the writing position of a laser beam on the photoreceptor drum 1Y. Asregards the other image forming units 10M and 10C, the same processingis done. By doing this way, the positional deviations of the opticalsystem components such as the fθ lens 36 in the image forming units 10Y,10M, 10C, and 10K from the optimum positions can be removed.

In this example, in order to correct the binarization reference value(also called the control reference value) for detecting the positions ofcolor registration marks, patch marks are previously formed on theintermediate transfer belt 6 through the image forming unit 10Y, 10M,10C, or 10K. A binarization reference value is a threshold level ofbinarization for use in the detection of the passage timing of a colorregistration mark formed on the intermediate transfer belt 6, and ishereinafter referred to as a threshold voltage Lth.

FIG. 5 is a perspective view showing an example of the arrangement ofthe toner image density sensor 11 and the registration sensors 12A and12B. In FIG. 5, the registration sensors 12A and 12B are provided abovethe portions near both the edges of the intermediate transfer belt 6. Inthe upstream side of the registration sensor 12A, the toner imagedensity sensor 11 is disposed. The toner image density sensor 11 and theregistration sensor 12A are arranged serially (side by side) at thespecified positions with respect to the running direction of theintermediate transfer belt 6. This is done for the purpose ofcalibrating the registration sensor 12 on the basis of a densitydetection signal S1 within one revolution period of the intermediatetransfer belt 6. That is, the control device 15 controls, for example,the image forming unit 10K to form patch marks Pm for color densitycorrection, namely, the patches (1) to (4) having different densitiesrespectively on the intermediate transfer belt 6 beforehand.

Further, during one revolution of the intermediate transfer belt 6, thedensity of the patch marks Pm formed is detected by the toner imagedensity sensor 11, and density correction control for adjusting thedensity of mark images is practiced, while the density of the patchmarks formed on the intermediate transfer belt 6 is detectedsuccessively also by the registration sensors 12A and 12B. After that,the densities of the color registration marks for the registration ofcomponent color images are adjusted on the basis of the densities of thepatch marks Pm.

FIG. 6(A)-1 to FIG. 6(B) are a drawing showing the patch marks Pm and awaveform drawing showing an example of density detection by theregistration sensor 12A or the like. FIG. 7(A) and FIG. 7(B) aredrawings showing waveform examples of a density detection signal S2′ bythe registration sensor 12A or the like. In any one of FIG. 6(B), FIG.7(A), and FIG. 7(B), the abscissa represents time t, and the ordinaterepresents the signal level of a density detection signal S2′ by theregistration sensor 12A or the like.

By the above-mentioned image forming unit 10Y, 10M, 10C, or 10K, severalkinds of patch mark Pm having different densities are formed on theintermediate transfer belt 6. The solid line shown in FIG. 6(B) shows awaveform of the density detection of the 4 patch marks Pm havingdifferent densities. In the case where the density of the patch mark Pmis low, as shown in FIG. 7(A), the waveform of the density detectionsignal S2′ is sharp, and its half-value width w1 is narrow. In the casewhere the density of the patch mark Pm is high, as shown in FIG. 7(B),the waveform of the density detection signal S2′ is dull, and itshalf-value width is broad.

In this example, the maximum value (MAX) of the density detection signalin the formation area of the patch marks Pm outputted from theregistration sensor 12A or the like and the minimum value (MIN) in thenon-formation area of the patch marks are detected by the control device15. The density detection signal S2′ in the non-formation area of thepatch marks Pm represents an output level reflecting the substrate,namely, the surface of the intermediate transfer belt. Then, the maximumvalue of the density detection signal S2′ concerning the non-formationarea of the patch marks Pm and the minimum value of it concerning theformation area of the patch marks Pm are removed from the object ofdetection. In this example, concerning the density detection signal S2′in the formation area of the patch marks Pm, the density giving thehighest signal level is detected out of the parts of the densitydetection signal S2′ for 4 patch marks Pm, patch (1) to patch (4).

In the example shown in FIG. 6(A)-1 to FIG. 6(B), the density of thepatch (1) is lowest and the density of the patch (4) is highest. In thecase where the density of the patch marks Pm fluctuates, in either ofthe cases of low density and high density, it sometimes occurs that thereflected light quantity becomes higher in relation to the condition ofthe substrate. In this example, the output of patch (1)>the output ofpatch (2)>the output of patch (4)>the output of patch (3), that is, theoutput of patch (3) is lowest. The patch marks Pm may have any patternshape so long as they can be detected by the color registration sensors12A etc.

FIG. 8 is a waveform drawing showing an example of the setting of athreshold voltage based on the density detection of patch marks Pm bythe registration sensor 12A or the like. In FIG. 8, the abscissarepresents time t, and the ordinate represents the signal level of adensity detection signal S2′ by the registration sensor 12A or the like.

The solid line in FIG. 8 shows the waveform at the time the density of 4patch marks Pm having different density values is detected. In FIG. 8,the average value is calculated on the basis of the maximum value (MAX)of the density detection signal S2′ and the minimum value (MIN) of it,and the average value is set as the threshold voltage Lth for theregistration sensors 12A etc. In this way, by setting the thresholdvoltage Lth to be the center of the two output levels, the stability ofdetection can be raised. This threshold voltage Lth is used in detectingthe passage timing at the time a color registration mark formed on theintermediate transfer belt 6 passes under the registration sensor 12A orthe like.

FIG. 9 is a perspective view showing an example of the detection ofcolor registration marks CR by the registration sensors 12A and 12B. InFIG. 9, after the density detection of the patch marks, the imageforming units 10Y, 10M, 10C, and 10K are controlled to form, during thenext one revolution of the intermediate transfer belt 6, for example,7-shaped color registration marks CR having its density adjusted. Thepositions of the color registration marks CR formed on the intermediatetransfer belt 6 are detected by the registration sensors 12A and 12B.Then, the control device 15 practices a color registration control foradjusting the forming positions of component color images on the basisof the positions of the color registration marks CR.

FIG. 10(A) and FIG. 10(B) are drawings showing an example ofbinarization of a position detection signal S2 by the registrationsensor 12A or the like. In FIG. 10(A), a position detection signal S2obtained by the registration sensor 12A or the like is binarized on thebasis of the threshold voltage Lth calculated in FIG. 8. In thisexample, a passage timing pulse signal Sp rises at the time ta when theposition detection signal S2 crosses the threshold voltage Lth at thepoint “a” during the decreasing of the position detection signal S2, andthe passage timing pulse signal Sp falls at the time tb when theposition detection signal S2 crosses the threshold voltage Lth at thepoint “b” during the increasing of the position detection signal S2.This passage timing pulse signal Sp is outputted from the comparator 59through the mask generation circuit 515 to the latch circuit 56, and isused as the reference for adjusting the positional deviations ofcomponent color images, for the purpose of calculating the amounts ofthe deviations of the writing positions of image components of thecolors Y, M, and C with respect to the writing position of the imagecomponent of the color BK.

Next, a first image forming method will be explained with reference toan example of operation of the color image forming apparatus 100. FIG.11 is a flow chart showing an example of operation of the color imageforming apparatus 100.

This example is premised on it that the intermediate transfer belt 6 isprovided in the image transfer system I, and before a color image isformed on the intermediate transfer belt 6 through the superposition ofcomponent color images on the basis of arbitrary image information, theforming positions of the component color images are adjusted on thebasis of the positions of color registration marks CR. Further, it istaken for example the case where the threshold voltage Lth in detectingthe positions of color registration marks CR is corrected in real timebefore the adjustment of the forming positions of component colorimages. The toner image density sensor 11 (the first detection system)and the registration sensors 12A and 12B (the second detection system)are arranged above the circumference of the intermediate transfer belt6, with the latter tow sensors 12A and 12B located at positions of thesame phase next to the toner image density sensor 11 with respect to therunning direction of the intermediate transfer belt 6.

With this incorporated into the image forming condition, in the steps A1to A3 of the flow chart shown in FIG. 11, the initial adjustmentconcerning the detection by the registration sensors 12A and 12B iscarried out, and after that, in the steps A4 to A8, the adjustment ofwriting position is made. In this initial adjustment, an optimumthreshold voltage Lth is determined from the sensor output of thesubstrate and the sensor output of the marks for density detection.

In this example, in the step A1, patch marks for the color densitycorrection are formed on the intermediate transfer belt 6. At this time,by the image forming unit 10Y, 10M, 10C, or 10K, several kinds of patchmark Pm having different density values as shown in FIG. 5 are formed onthe intermediate transfer belt 6. After that, the procedure moves to thestep A2, where the density of the patch marks Pm formed on theintermediate transfer belt 6 is detected by the registration sensors 12Aetc. For example, a density detection signal S2′ detected by theregistration sensor 12A is such one as shown in FIG. 6(B).

Further, the procedure moves to the step A3, where the threshold voltageLth for the position detection of color registration marks CR iscorrected on the basis of the density detection signal S2′ of the patchmarks outputted from the registration sensor 12A. The threshold voltageLth is corrected by the operation shown in FIG. 8. At this time, in thecontrol device 15, the maximum value (MAX) of a density detection signalS2′ of the non-formation area of the patch marks Pm outputted from theregistration sensor 12A or the like and the minimum value (MIN) of itsformation area are detected, and the average value is calculated on thebasis of the maximum value and the minimum value of the densitydetection signal S2′. A density detection signal S2′ in thenon-formation area of patch marks Pm reflects the substrate, that is,the surface of the intermediate transfer belt 6.

This average value is determined to be the threshold voltage Lth for thedetection of the registration marks by the registration sensors 12.Further, the density values of color registration marks CR for theregistration of component color images are adjusted on the basis of thedensity of the patch marks Pm. By forming the color registration marksCR on the basis of the density of the patch marks Pm detected in theabove, it is possible to make the density of color registration marks CRformed most suitable for the position detection by the registrationsensors 12. Further, toner consumption amount and adjustment time forthe formation of color registration marks can be reduced.

Then, color registration marks CR (mark images) having their densityadjusted are formed on the intermediate transfer belt 6 in the step A4.After that, the positions of the color registration marks CR formed onthe intermediate transfer belt 6 are detected by the registrationsensors 12 in the step A5. At this time, the passage timing of a colorregistration mark is detected on the basis of the threshold voltage Lthshown in FIG. 10(A). After that, in the step 6, the amounts of colordeviations are calculated on the basis of passage timings, for thepurpose of adjusting the forming positions of component color images onthe basis of the positions of the color registration marks CR.

Then, the procedure moves to the step A7, where the amounts of colordeviations are compared with the target value. If the amounts of colordeviations are not greater than the target value, the processing iscompleted without adjusting the forming positions of component colorimages. If the amounts of color deviations exceed the target value, theprocedure moves to the step A8, where color deviation correction ismade. In this color deviation correction, for example, in the correctionmeans for the Y-color 5Y, the fθ lens adjusting mechanism 42 is drivenon the basis of a position correction signal Sy (YVV), and the lensholding mechanism 41 is moved to rotate the fθ lens about its opticalaxis perpendicular to the image formation surface for adjustment. Bydoing this, the writing position of a laser beam on the photoreceptordrum 1Y can be adjusted.

Then, the procedure moves back to the step A4, and the above-mentionedprocessings are repeated, for the purpose of making the amounts of colordeviations zero to adjust the forming positions of component colorimages to the optimum. After that, by the image forming units 10Y, 10M,10C, and 10K having the image forming position adjusted to the optimum,a color image can be formed on the intermediate transfer belt 6 in thesame way as conventional methods.

As explained in the above, according to the color image formingapparatus 100 and the image forming method as the embodiment 1 of thepresent invention, it can be put into practice that the density of patchmarks Pm for color density correction is detected by the registrationsensors 12, and the threshold voltage Lth for detecting the positions ofthe color registration marks CR is corrected on the basis of the densitydetection signals S2′ of the patch marks Pm outputted from saidregistration sensors 12.

Accordingly, it is possible to make a correction such that the thresholdvoltage Lth for detecting the positions of color registration marks CRis adapted for the state of the intermediate transfer belt 6 or theimage forming units 10Y, 10M, 10C, and 10K in accordance with thecondition of use. On top of it, because the density of colorregistration marks can be optimized, a high accuracy in the detectionprocessing of color registration marks can be secured.

Owing to this, even if the condition of use changes with the passage oftime due to the change of reflection light quantity at the intermediatetransfer belt 6 and the decrease of the light emission quantity of thesensors, the proper positions of color registration marks can be exactlydetected; therefore, the forming positions of component color images canbe adjusted at a high accuracy on the basis of a high-reliabilityposition detection signal S2. Accordingly, because component colorimages can be exactly superposed on the intermediate transfer belt 6, itis possible to transfer a color image on a desired paper sheet P at ahigh accuracy.

(2) Embodiment 2

FIG. 12 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 200 as the embodiment 2 of the present invention.

In this example of the embodiment, it is put into practice that, inorder that a color image may be formed through the superposition ofcomponent color images on the basis of arbitrary image information,there is provided a control device 15 for controlling an intermediatetransfer belt 6 and image forming units 10Y, 10M, 10C, and 10K on thebasis of reversed mark images as the reversed ones of mark images forthe registration of component color images, and at least, reversed markimages are formed beforehand on the intermediate transfer belt 6, andafter that, the forming positions of component color images are adjustedon the basis of the position detection of mark images defined by thevoid portions of these reversed mark images. Thus, even if scratchesetc. are produced on the intermediate transfer belt 6 due to the changewith the passage of time caused by maintenance operations and wear ofparts, a color image can be transferred onto a desired paper sheet P ata high accuracy.

The color image forming apparatus 200 shown in FIG. 12 is an apparatusfor forming a color image through the superposition of component colorimages on the basis of arbitrary image information. Said apparatus 200comprises the intermediate transfer belt 6, and operates to transfer acolor image onto a desired paper sheet P. Along this intermediatetransfer belt 6, the image forming units 10Y, 10M, 10C, and 10K areprovided to practice the formation of a color image. Registrationsensors 12 as an example of the detection means are provided, forexample, at the left side with respect to the intermediate transfer belt6 shown in the drawing, and are given a role to detect the positions ofmark images formed on the intermediate transfer belt 6.

The registration sensors 12 are connected to the control device 15, bywhich the intermediate transfer belt 6 and the image forming units 10Y,10M, 10C, and 10K are controlled on the basis of the output of theseregistration sensors 12. The control device 15 controls the imageforming units 10Y, 10M, 10C, and 10K in such a way that at leastreversed color registration marks RCR as the reversed ones of markimages for the registration of component color images are previouslyformed on the intermediate transfer belt 6, and the forming positions ofcomponent color images are adjusted on the basis of the positiondetection of mark images defined by the void portions of the reversedcolor registration marks RCR. For example, by the control device 15, onthe basis of the output of the registration sensors 12A etc., with thereversed color registration mark RCR for the color BK taken as thereference, the image forming units 10Y, 10M, and 10C for the othercolors Y, M, and C are controlled. By this control, the writingpositions for the colors Y, M, and C are adjusted to come to agree withthe writing position for the color BK.

A storage device 14 is connected to the control device 15, and aplurality of kinds of reversed mark image information (hereinafterreferred to as reversed mark image data DP) for reversing mark imagesfor the registration of component color images are stored in it. Ofcourse, the way of handling the data is not restricted to this, and alsoit is appropriate that mark image information for forming mark imagesfor the registration of component color images is stored in the storagedevice 14 beforehand, and at the time of detecting color registrationmarks, reversed mark image data DP are prepared on the basis of the markimage information and the pattern width. This is done for the purpose offorming reversed color registration marks RCR obtained as the reversedones of mark images on the basis of reversed mark image data DP on theintermediate transfer belt 6.

In this example, a developing device 4Y for forming a toner image of thecolor Y on the intermediate transfer belt 6 is provided in the imageforming unit 10Y, a developing device 4Y for forming a toner image ofthe color M on the intermediate transfer belt 6 is provided in the imageforming unit 10M, a developing device 4C for forming a toner image ofthe color M is provided in the image forming unit 10C, and a developingdevice 4K for forming a toner image of the color BK is provided in theimage forming unit 10K; toner images formed on the intermediate transferbelt 6 by these developing devices 4Y, 4M, 4C, and 4K on the basis ofthe reversed mark image data DP for the colors Y, M, C, and BK form thereversed registration marks RCR of the respective colors, and the voidportions with no toner particles deposited represent the mark images. Inaddition, as regards items having the same name and sign as thoseexplained in the embodiment 1, the explanation will be omitted becausethey have the same function.

FIG. 13(A) and FIG. 13(B) are conceptual drawing showing an example ofthe structure of a mark image and a reversed mark image having the samepattern as that used in the first image forming apparatus 100. To takethe running direction of the intermediate transfer belt 6 as thesub-scanning direction, and the direction perpendicular to thissub-scanning direction as the main scanning direction, the registrationmark CR is made up of a line segment parallel to the main scanningdirection and an oblique line segment not perpendicular to the mainscanning direction meeting together at a point. This shape is devisedfor the purpose of detecting the positional deviation in the mainscanning direction from the timings when the above-mentioned two linesegments of the color registration mark pass under the registrationsensor 12.

The reversed color registration mark RCR shown in FIG. 13(B) is anexample of the reversed mark image; this is the rectangular-shapedpattern surrounding the mark image formed of the void portion. Thisreversed registration mark RCR is applied to the second image formingapparatus 200 explained with reference to FIG. 2. In FIG. 13(B), Wmdenotes the length of the pattern of the reversed color registrationmark RCR in the sub-scanning direction, and is the range of toner imageformation in the sub-scanning direction. Ws denotes the pattern width ofthe reversed color registration mark RCR in the main scanning direction,and is the range of toner image formation in the main scanningdirection.

According to the method of forming the color registration mark shown inFIG. 13(A), it is considered that if belt scratches or the like arepresent on the part where no toner particle is deposited due to thechange of the intermediate transfer belt 6 with the passage of time,sometimes an erroneous detection is made due to a noise caused by thebelt scratches by the registration sensor 12A or the like. Therefore, itis made possible to cover belt scratches by using reversed registrationmarks of a method of the present invention shown in FIG. 13(B).

In this example, a position detection signal S2 of a mark image definedby the void portion of the reversed color registration mark RCR isoutputted from the registration sensor 12. The position detection signalS2 is obtained by the detection of the edge of the void portion, and inorder to carry out image processing in the same way as the case ofnon-reversed mark images, only it is necessary to reverse the edgedetection logic of this position detection signal S2.

To state it concretely, it is practiced to connect an inverter to theoutput of the registration sensor 12 to invert the signal logic, andmask the area except the reversed mark formation area by the use ofhardware for the position detection signal S2. This is done for thepurpose of detecting the portion not covered with toner particles, thatis, the position of the mark image at a high accuracy, in the case wherethe range uniformly covered with toner particles is limited in thesub-scanning direction on the intermediate transfer belt 6.

In this example, reversed color registration marks RCR are formed tohave arbitrary pattern widths Ws in the main scanning direction; thepattern widths of the reversed color registration marks RCR are variedarbitrarily for the purpose of reducing the toner consumption for thedetection of color registration marks.

FIG. 14 is a conceptual drawing showing an example of formation ofreversed color registration marks RCR for the colors BK, C, M, and Y.

The reversed color registration mark RCR for the color BK (hereinafterreferred to as the BK-color reversed pattern PK simply), the reversedcolor registration mark RCR for the color C (hereinafter referred to asC-color reversed pattern PC simply), the reversed color registrationmark RCR for the color M (hereinafter referred to as M-color reversedpattern PM simply), and the reversed color registration mark RCR for thecolor Y (hereinafter referred to as Y-color reversed pattern PY simply)shown in FIG. 14 are an example of patterns formed serially in thesub-scanning direction on the intermediate transfer belt 6.

The case where the reversed patterns PK, PC, PM, and PY are detected bythe two registration sensors 12A and 12B is shown. With respect to therunning direction of the intermediate transfer belt 6, the registrationsensor 12A is provided above a position near the right edge of the belt6, and the registration sensor 12B is provided above a position near theleft edge of the belt 6. The broken lines represent the apparent loci ofthe two registration sensors due to the revolution of the intermediatetransfer belt 6.

In this example of patterns, the two BK-color reversed patterns PK areintegrally formed in the main scanning direction; the mark portions areformed as the void portions in a toner image which is uniform over thewhole width, and are detected by the registration sensors 12A and 12Brespectively. If all the reversed patterns are formed in a uniform tonerimage over the whole width in the main scanning direction in theabove-mentioned way, toner consumption increases. Therefore, it isappropriate to form a uniform toner image in the narrow area near themark portion, to limit the pattern width Ws like the reversed patternsPC, PM, and PY for the colors C, M, and Y respectively. Thus, the tonerconsumption can be suppressed.

In the example of the reversed pattern PM for the color M, the patternwidth Ws of the reversed pattern PM is made narrower than the width ofthe mark image. In this case, the toner consumption can be suppressed tothe minimum. The reversed pattern PM has a structure such that aplurality of partial figure patterns partitioning the void portions arearranged, and is different from the reversed patterns PK, PC, and PY forthe colors BK, C, and Y respectively having a rectangular patternstructure surrounding the whole of the mark images formed of the voidportions.

Reversed mark image data DP for forming these plural kinds of reversedpattern, PY, PM, PC, and PK are stored in the storage device 14. It isappropriate to select the reversed patterns PY, PM, PC, and PK inaccordance with the state of use of the intermediate transfer belt 6.

Further, in the case where two or more kinds of reversed colorregistration mark are formed serially in the sub-scanning direction onthe intermediate transfer belt 6, the image forming units 10Y, 10M, 10C,and 10K are controlled by the control device 15 in such a way as to makethe lower edge portion of a reversed color registration mark of one kindformed on the intermediate transfer belt 6 overlap the upper edgeportion of a reversed color registration mark of another kind, forexample, by one pixel.

In this way, as in the example of the reversed pattern PM for the colorM and the reversed pattern PY for the color Y shown in FIG. 14,concerning the range to be covered uniformly with toner particles on theintermediate transfer belt 6, it is possible to make the toner image forthe color M overlap the toner image for the color Y. By doing this way,on top of it that the reading of mark images by the use of hardware canbe easily limited, the density of a color image after the superpositionof the component color images can be also confirmed.

Next, the second image forming method will be explained with referenceto an example of the operation of the second color image formingapparatus 200. FIG. 15 is a flow chart showing an example of theoperation of the color image forming apparatus 200.

This example of the embodiment relates to a case where a color image isformed through the superposition of the component color images on theintermediate transfer belt 6 on the basis of arbitrary imageinformation, and reversed mark image data DP for reversing mark imagesfor the registration of the component color images are preparedbeforehand. The reversed mark image data DP are read out from thestorage device 14 such as a ROM. Of course, it is also appropriate toprepare reversed mark image data DP on the basis of the mark imageinformation and the pattern width at the time of detecting colorregistration marks. Now, take it for instance the case where colordeviations are corrected in the order the color C, M, and Y on the basisof the color BK. The correction of color deviations are made in such away that the writing positions for the colors Y, M, and C are modifiedon the basis of the writing position for the color BK.

With this incorporated into the image formation condition, reversedcolor registration marks RCR of one of the component colors are formedon the basis of reversed mark image data DP for the color concerned inthe step B1 of the flow chart shown in FIG. 15. In this example, aBK-color reversed pattern PK is formed at first on the photoreceptordrum 1K by the image forming unit 10K. After that, the procedure movesto the step B2, where the reversed color registration mark RCR istransferred from the photoreceptor drum onto the intermediate transferbelt 6. In this example, the BK-color reversed pattern PK is transferredfrom the photoreceptor drum 1K to form the BK-color reversed pattern PKon the intermediate transfer belt 6.

In this example, when a toner image is formed on the intermediatetransfer belt 6 on the basis of reversed mark image data DP, the portionof the toner image formed on the intermediate transfer belt 6 makes upthe BK color reversed pattern PK (reversed color registration mark), andthe void portions not covered with toner particles make up the markimages. Then, it is practiced to detect the position of the mark imagesdefined by the void portions in the BK-color reversed pattern PK by theregistration sensors 12A etc. in the step B3.

Further, in the step B4, it is practiced to calculate the correctionvalue for the BK color deviation on the basis of the positions of themark images defined by the void portions in the control device 15. Afterthat, the procedure moves to the step B5, whether or not color deviationcorrection is to be practiced is judged by the control device 15.Whether or not color deviation correction is to be practiced is judgedby comparing the deviation value with a control target value determinedbeforehand.

If the color deviation amount exceeds the target value and a colordeviation correction is required, the procedure moves to the step B6,where the image writing section 3K is controlled by the control device15. In this color deviation correction, in the correction means for theBK-color 5K, the fθ lens adjusting mechanism 42 is driven on the basisof a position correction signal Sy (YVV), and the lens holding mechanism41 is moved to rotate the fθ lens about its optical axis perpendicularto the image formation surface for adjustment. By doing this, thewriting position of a laser beam on the photoreceptor drum 1K can beadjusted.

If the color deviation amount is not greater than the target value andno color deviation correction is required in the step B5, the proceduremoves to the step B7, and whether or not the detection of colorregistration marks for the other colors is to be practiced is judged.Because the detection of color registration marks for the other colors,namely, for the colors C, M, and Y, is to be practiced, the proceduremoves back to the step B1.

Then, in the step B1, C-color reversed patterns PC are formed on thephotoreceptor drum 1C by the image forming units 10C on the basis ofreversed mark image data DP, in the step B2, the C-color reversedpatterns PC are transferred from the photoreceptor drum 1C onto theintermediate transfer belt 6, and in the step B3, the positions of themark images defined by the void portions in the C-color reversedpatterns PC are detected by the registration sensors 12A etc.

Further, in the step B4, it is practiced to calculate the correctionvalue of the color deviation amount on the basis of the positions of themark images defined by the void portions in the control device 15. Atthis time, in the control device 15, the writing position of thereversed color registration mark RCR for the color BK and the writingposition of the reversed color registration mark RCR for the color C aredetected, and the correction value is calculated from the amount ofdeviation in the case where the writing position of the reversed colorregistration marks RCR for the color C is converted into the writingposition of the reversed color registration marks RCR for the color BK.

After that, the procedure moves to the step B5, and whether or not acolor deviation correction is to be practiced is judged by the controldevice 15. Whether or not a color deviation correction is to bepracticed is judged by comparing the deviation amount with a controltarget value determined beforehand in the same way as the case of thecolor BK. If the color deviation amount exceeds the target value, and acolor deviation correction is required, the procedure moves to the stepB6, where the image writing section 3C is controlled by the controldevice 15. In this color deviation correction, in the correction means5C for the color C, the fθ lens adjusting mechanism 42 and the opticalaxis adjusting mechanism 43 are driven on the basis of a positioncorrection signal Sc (CVV), and the lens holding mechanism 41 is movedto rotate the fθ lens about its optical axis perpendicular to the imageformation surface for adjustment. By doing this, the writing position ofa laser beam on the photoreceptor drum 1C can be adjusted.

Then, in the step B1, M-color reversed patterns PM are formed on thephotoreceptor drum 1M by the image forming units 10M on the basis ofreversed mark image data DP, in the step B2, the M-color reversedpatterns PM are transferred from the photoreceptor drum 1M onto theintermediate transfer belt 6, and in the step B3, the positions of themark images defined by the void portions in the M-color reversedpatterns PM are detected by the registration sensors 12A etc.

Further, in the step B4, it is practiced to calculate the correctionvalue of deviation amount for the color M on the basis of the positionsof the mark images defined by the void portions in the control device15. At this time, in the control device 15, the writing position of thereversed color registration marks RCR for the color BK and the writingposition of the reversed color registration marks RCR for the color Mare detected, and the correction value is calculated from the amount ofdeviation in the case where the writing position of the reversed colorregistration marks RCR for the color M is converted into the writingposition of the reversed color registration marks RCR for the color BK.

After that, the procedure moves to the step B5, and whether or not acolor deviation correction is to be practiced is judged by the controldevice 15. Whether or not a color deviation correction is to bepracticed is judged by comparing the deviation amount with a controltarget value determined beforehand.

If the color deviation amount exceeds the target value, and a colordeviation correction is required, the procedure moves to the step B6,where the image writing section 3M is controlled by the control device15. In this color deviation correction, in the correction means 5M forthe color M, the fθ lens adjusting mechanism 42 is driven on the basisof a position correction signal Sm (MVV), and the lens holding mechanism41 is moved to rotate the fθ lens about its optical axis perpendicularto the image formation surface for adjustment. By doing this, thewriting position of a laser beam on the photoreceptor drum 1M can beadjusted.

Further, in the step B1, Y-color reversed patterns PY are formed on thephotoreceptor drum 1Y by the image forming units 10C on the basis ofreversed mark image data DP, in the step B2, the Y-color reversedpatterns PY are transferred from the photoreceptor drum 1Y onto theintermediate transfer belt 6, and in the step B3, the positions of themark images defined by the void portions in the Y-color reversedpatterns PY are detected by the registration sensors 12A etc.

Further, in the step B4, it is practiced to calculate the correctionvalue of the color deviation amount on the basis of the position of themark images defined by the void portions in the control device 15. Atthis time, in the control device 15, the writing position of thereversed color registration marks RCR for the color BK and the writingposition of the reversed color registration marks RCR for the color Yare detected, and the correction value is calculated from the amount ofdeviation in the case where the writing position of the reversed colorregistration marks RCR for the color Y is converted into the writingposition of the reversed color registration marks RCR for the color BK.

After that, the procedure moves to the step B5, and whether or not acolor deviation correction is to be practiced is judged by the controldevice 15. Whether or not a color deviation correction is to bepracticed is judged by comparing the deviation amount with a controltarget value determined beforehand.

If the color deviation amount exceeds the target value, and a colordeviation correction is required, the procedure moves to the step B6,where the image writing section 3Y is controlled by the control device15. In this color deviation correction, in the correction means 5Y forthe color Y, the fθ lens adjusting mechanism 42 is driven on the basisof a position correction signal Sy (YVV), and the lens holding mechanism41 is moved to rotate the fθ lens about its optical axis perpendicularto the image formation surface for adjustment. By doing this, thewriting position of a laser beam on the photoreceptor drum 1Y can beadjusted.

As explained in the above, according to a color image forming apparatusand an image forming method as the embodiment 2 of the presentinvention, it is practiced that, by the control device 15, a BK-colorreversed pattern PK is formed beforehand on the intermediate transferbelt 6, and after that, the image writing section 3C is controlled so asto adjust the position of forming the component image of the color C onthe basis of the position of the mark images defined by the voidportions of the C-color reversed patterns PC formed on the intermediatetransfer belt 6. Also as regards the color M and Y, it is practiced thatthe image writing section 3M and the image writing section 3Y arecontrolled so as to adjust the forming positions of the component imagesof the colors M and Y.

Accordingly, because the area except the void portions defining the markimages can be covered with reversed color registration marks (tonerimages) of the colors Y, M, C, and BK, even if scratches etc. areproduced on the intermediate transfer belt 6 due to the change with thepassage of time caused by the maintenance operations and wear of parts,the proper positions of the mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted at a high accuracy on the basis of a high-reliability positiondetection signal S2 with no noise signal due to scratches etc.superposed. On top of it, for the calculation of color deviation values,the structure based on a conventional method can be used as it is withalmost no alteration. Further, because the reading of portions otherthan reversed color registration marks RCR is excluded by the use ofhardware, and on top of it, toner consumption can be suppressed.Accordingly, because component color images can be superposed exactly onthe intermediate transfer belt 6, a color image can be transferred ontoa desired paper sheet P without being influenced by the change with thepassage of time.

(3) Embodiment 3

FIG. 16 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 300 as the embodiment 3 of the present invention.

In this example of the embodiment, there is provided a control device 15for controlling an intermediate transfer belt 6 and image forming units10Y, 10M, 10C, and 10K on the basis of the position detection of markimages, mark images or reversed registration marks RCR obtained byreversing said mark images for the registration of component colorimages are formed on the intermediate transfer belt 6, and the imageforming units 10Y, 10M, 10C, and 10K are controlled so as to adjust theforming positions of component color images on the basis of the positiondetection of non-reversed mark images or mark images defined by the voidportions of reversed color registration marks formed on the intermediatetransfer belt 6.

Owing to this, in the case where a new intermediate transfer belt 6 isused for the first time, or in the case where an old intermediatetransfer belt 6 is replaced with a new one, the forming positions ofcomponent color images can be adjusted on the basis of the positiondetection of mark images. On the other hand, even if scratches etc. areproduced on the intermediate transfer belt 6 due to the change with thepassage of time caused by maintenance operations, wear of parts, etc.,the forming positions of component color images can be adjusted at ahigh accuracy on the basis of a high-reliability position detectionsignal S2 with no noise signal due to scratches etc. superposed.

The color image forming apparatus 300 shown in FIG. 16 is an apparatusfor forming a color image through the superposition of the componentcolor images on the basis of arbitrary image information. Said imageforming apparatus 300 comprises the intermediate transfer belt 6, and isgiven a function to transfer a color image onto a desired paper sheet P.Along this intermediate transfer belt 6, there are provided the imageforming units 10Y, 10M, 10C, and 10K given a function to form a colorimage. A plurality of registration sensors 12 which are an example ofthe detection means are disposed above the intermediate transfer belt 6,and are given a role to detect the positions of the mark images formedon the intermediate transfer belt 6.

The control device 15 is connected to the registration sensors 12, andcontrols the intermediate transfer belt 6 and the image forming units10Y, 10M, 10C, and 10K on the basis of the output of these registrationsensors 12. The control device 15, at least, in accordance with thestate of use of the intermediate transfer belt 6, controls the imageforming unit 10Y, 10M, 10C, and 10K in a way such that mark images orreversed color registration marks obtained by reversing said mark imagesfor the registration of component color images are formed on theintermediate transfer belt 6, and the positions of forming componentcolor images are adjusted on the basis of the position detection of thecolor registration marks CR or the color registration marks CR definedby the void portions of the reversed color registration marks RCR formedon the intermediate transfer belt 6.

A storage device 14 is connected to the control device 15, and is givena role to store mark image information for forming color registrationmarks CR and reversed mark image data DP for forming reversed colorregistration marks RCR. Of course, the way of handling the informationis not limited to this, and also it is appropriate that mark imageinformation for forming mark images for the registration of componentcolor images are stored in the storage device 14, and at the time of thedetection of color registration marks CR, reversed mark image data DPare prepared on the basis of the mark image information and the patternwidth, in order to form reversed color registration marks RCR as thereversed ones of the mark images on the intermediate transfer belt 6 onthe basis of the reversed mark image data DP.

In this example, with the running direction of the intermediate transferbelt 6 taken as the sub-scanning direction and the directionperpendicular to the sub-scanning direction taken as the main scanningdirection, the plural registration sensors 12 are arranged in the mainscanning direction. By doing this, the surface state of the color imageformation surface of the intermediate transfer belt 6 (hereinafterreferred to as the registration mark formation area) can be detectedwith respect to its plural divisional parts for the respective sensors.Accordingly, for each of the divisional registration mark formationareas extending in the sub-scanning direction to be detected by theconcerned registration sensor 12, mark image information or reversedmark image information for forming color registration marks CR orreversed color registration marks RCR can be selected from the storagedevice 14.

In this example, a developing device 4Y for forming a toner image of thecolor Y on the intermediate transfer belt 6 is provided in the imageforming unit 10Y, a developing device 4M for forming a toner image ofthe color M is provided in the image forming unit 10M, a developingdevice 4C for forming a toner image of the color C is provided in theimage forming unit 10C, and a developing device 4K for forming a tonerimage of the color BK is provided in the image forming unit 10K; tonerimage portions of the colors Y, M, C, and BK formed on the intermediatetransfer belt 6 make up the reversed registration marks RCR of therespective colors, and the void portions where no toner particle existsrepresent the mark images. In addition, as regards items having the samename and sign as those explained in the embodiment 1, the explanationwill be omitted because they have the same function.

The control device 15 detects the surface state of the registration markformation area of the intermediate transfer belt 6 by means of theregistration sensors 12, and judges whether or not the surface state ofthe intermediate transfer belt 6 is good on the basis of the output ofthe registration sensors 12 to control the selection and reading ofinformation in the storage device 14. In this example, the registrationsensors 12, the storage device 14, and the control device 15 make up amark detection judging means 80.

In the mark detection judging means 80, the surface state of theregistration mark formation area of the intermediate transfer belt 6 isdetected, and in the control device 15, a detection unable flag FG isprepared. This detection unable flag FG is used as the basis of thejudgment concerning whether or not the registration sensors 12 will makean erroneous detection at the time of the detection of colorregistration marks.

In this example, the control device 15 receives the detection output ofthe registration sensors 12, and controls the image forming units 10Y,10M, 10C, and 10K so as to form color registration marks CR or reversedcolor registration marks RCR as the reversed ones of said colorregistration marks CR on the registration mark formation area inaccordance with the surface state of the intermediate transfer belt 6.

Now, let it be the first adjustment mode, a mode in which it ispracticed the processing of forming color registration marks CR on theintermediate transfer belt 6 on the basis of mark image information(hereinafter referred to also as the first mark producing method), andadjusting the forming positions of component color images on the basisof the position detection of the color registration marks CR formed onthe intermediate transfer belt 6. Further, let it be the secondadjustment mode, a mode in which it is practiced the processing offorming reversed color registration marks RCR on the intermediatetransfer belt 6 on the basis of reversed mark image data DP (hereinafterreferred to also as the second mark producing method), and adjusting theforming positions of component color images on the basis of the positiondetection of the marks images defined by the void portions of thereversed color registration marks RCR formed on the intermediatetransfer belt 6.

To this control device 15, an operation means 18 and a display device 29are connected, and an operation to set (select) either the firstadjustment mode or the second adjustment mode is carried out. In thecontrol device 15, it is practiced to control the intermediate transferbelt 6 and the image forming units 10Y, 10M, 10C, and 10K on the basisof the output of the operation means 18. A setting screen at the time ofimage formation is displayed on the display device 29. A touch panelwith the operation means 18 incorporated is used for the display device29.

For example, if the surface state of the registration mark formationarea is good, or if the first adjustment mode is set, it is practicedthat color registration marks CR are formed on the intermediate transferbelt 6 by the first mark producing method, and the forming positions ofcomponent color images are adjusted on the basis of the positiondetection of the color registration marks CR formed on the intermediatetransfer belt 6.

Further, if the surface state of the registration mark formation area isinferior to a reference value determined beforehand, of if the secondadjustment mode is set, it is practiced that reversed color registrationmarks RCR are formed on the intermediate transfer belt 6 by the secondmark producing method, and the forming positions of component colorimages are adjusted on the basis of the position detection of the colorregistration marks CR defined by the void portions of the reversed colorregistration marks RCR formed on the intermediate transfer belt 6.

In this example, a fuse F for detecting a new belt is further providedat the intermediate transfer belt 6. This fuse F is connected to adiscrimination circuit 19. The discrimination circuit 19 is provided,for example, in the control device 15, operates to output a new beltdetection signal based on the melting-off or no melting-off of the fuseF, and melts off the fuse F on the basis of an externally set signal.

In this example, the control device 15 controls the image forming units10Y, 10M, 10C, and 10K so as to form color registration marks CR orreversed color registration marks RCR as the reversed ones of said colorregistration marks CR on the registration mark formation area of theintermediate transfer belt 6 on the basis of a new belt detection signalobtained from the discrimination circuit 19 for the intermediatetransfer belt 6.

Further, in the case where the control device 15 has already controlledthe image forming units 10Y, 10M, 10C, and 10K on the basis of thesecond adjustment mode, if a new belt detection signal Snd obtained fromthe discrimination circuit 19 indicates that “the intermediate transferbelt is new”, the control device 15 is made to switch over the mode fromthe second to the first, to control the image forming units 10Y, 10M,10C, and 10K. This is done for the purpose of switching over the markproducing method from the second to the first.

FIG. 17 is a block drawing showing an example of the internal structureof the positional deviation control system and the image formationcontrol system of the control device 15.

The control device shown in FIG. 17 is such one that further comprises,in addition to the positional deviation control system shown in FIG. 3,an image formation control system; as regards the image formationcontrol system, it is equipped with an image formation control section13 and the discrimination circuit 19, and as regards the positionaldeviation control system, it is equipped with a CPU 55, a RAM 57, alateral magnification correction section 510, a sub-scanning correctionsection 511, a main scanning correction section 512, a skew correctionsection 513, a mask generation circuit 515, and a signal detectioncircuit 516. The signal detection circuit 516 is composed of anoscillator 51, a frequency divider 52, a polygonal mirror drivingcircuit 53, a counter circuit 54, a latch circuit 56, a D/A converter58, and a comparator 59 as shown in FIG. 3.

The CPU 55 comprises a logic operation section 551, a deviation amountdetection section 552, and a correction value calculation section 553.In the logic operation section 551, it is practiced to read out datafrom the RAM 57 and make logic operations. In the deviation amountdetection section 552, it is practiced to detect color deviation amountsand output color deviation detection values. In the correction valuecalculation section 553, it is practiced to calculate correction valuessuch as delay control data D10 for correcting the writing position, VVgeneration control data D11, HV generation control data D12, and skewcorrection data D13 on the basis of color deviation detection values.

The delay control data D10 are outputted from the correction valuecalculation section 553 to the lateral magnification correction section510. The VV generation control data D11 are outputted from thecorrection value calculation section 553 to the sub-scanning correctionsection 511. The HV generation control data D12 are outputted from thecorrection value calculation section 553 to the main scanning correctionsection 512. The skew correction data D13 are outputted from thecorrection value calculation section 553 to the skew correction section513. In addition, as regards items having the same name and the samesign as those shown in FIG. 3, the explanation will be omitted.

In the image formation control system 13, arbitrary image informationDin is subjected to image processing, and image information for theY-color Dy is separated and outputted to the image writing section (theexposure means) 3Y. In the same way, it is practiced that imageinformation for the M-color Dm is outputted to the image writing section3M, image information for the C-color Dc is outputted to the imagewriting section 3M, and image information for the BK-color Dk isoutputted to the image writing section 3K.

To the image control system 13, the storage device 14, the operationmeans 18, and the discrimination circuit 19 shown in FIG. 16 areconnected. In the discrimination circuit 19, it is practiced that a newbelt detection signal Snd based on the melting-off or no melting-off ofthe fuse F provided at the intermediate transfer belt 6 is outputted andthe fuse F is melted off on the basis of an externally set signal Sop.

Now, let the level “L” denote the signal logic of a new belt detectionsignal Snd in the case where the intermediate transfer belt is new, andlet the level “H” denote the signal logic of a new belt detection signalSnd in the case where the intermediate transfer belt 6 has been used atleast once; then, in the discrimination circuit 19, in the case wherethe fuse F is not melted off, a new belt detection signal Snd of thelevel “L” is outputted to the image formation control system 13. In thecase where the fuse F is forcibly melted off on the basis of anexternally set signal Sop, a new belt detection signal Snd of the level“H” is outputted to the image formation control system 13. An externallyset signal Sop is made to be outputted through the operation means 18 tothe discrimination circuit 19.

FIG. 18(A) and FIG. 18(B) is a circuit diagram showing an example of thestructure of a new belt detection circuit 90. The new belt detectioncircuit 90 shown in FIG. 18(A) is an example of the discriminationcircuit 19, and comprises a pnp-type bipolar transistor TR, resistors R1and R2, and a diode D. The resistors R1 and R2 are serially connected,and these serially connected resistors R1 and R2 are connected betweenthe source line VCC and the ground line GND. The anode of the diode D isconnected to the connection point p between the resistors R1 and R2, andthe cathode is connected to the fuse F for the detection of a new belt.

Further, the emitter of the transistor TR is connected to the sourceline VCC, and the collector is connected to each of the fuse F and thecathode of the diode D. To the base of the transistor TR, an externallyset signal Sop for controlling the forcible opening is supplied. In thisexample, when an externally set signal Sop of the high level(hereinafter referred to as “H” level) is supplied to the base of thetransistor TR, the transistor TR turns on, to let an over current flowthrough the fuse F, and the fuse F is opened (melted off).

The electric potential at the series connection point p has the samelevel as the new belt detection signal Snd of the low level (hereinafterreferred to as the “L” level) at the time the fuse is not melted off(closed). When the fuse F is melted off, it comes to have the same levelas the new belt detection signal Snd of the “H” level, which is thedivision of the electric potential difference between the source lineVCC and the ground line GND by the resistors R1 and R2 at the point p.

Although a fuse F is used in this example, it is also appropriate tomemorized data indicating whether the belt unit is new or old in avolatile memory chip such as an EEPROM instead of it.

FIG. 19(A) to FIG. 19(C) are conceptual drawings showing examples ofdisplay on the operation screen at the time of image formation in thedisplay device 29. In this example, when the copy mode is selected bythe use of the operation means 18, an operation screen for keyoperations P1 as shown in FIG. 19(A) is displayed on the display device29. On this operation screen P1, a message such as “Copying possible.Set your document with front page upside.” is indicated, while softwareswitches such as “Copy setting”, “Copy reserving”, and “Reservationlist” are indicated. FIG. 19(A) shows the case where “Copy reserving” isselected.

When the software switch “Copy setting” is selected on this operationscreen P1, a memory setting mode screen as shown in FIG. 19(B) isdisplayed. The memory setting mode screen P2 is usually operated by aservice man or the like at the time of the installation of apparatus. Inthis memory setting mode screen, item keys such as “1 Software switchsetting”, “2 Paper size setting”, “3 PM count/cycle”, “4 Data cancel”,“5 Number of copies”, “6 Password setting”, “7 Telephone numbersetting”, “8 Indication of serial number”, “9 Indication of ROMversion”, and “10 KRDS setting” are indicated.

When the “1 Software switch setting” is selected out of these item keys,a software switch mode screen P3 as shown in FIG. 19(C) is displayed. Inthis software switch mode screen P3, an operation to set (select) eitherthe first adjustment mode or the second adjustment mode is done. This isa screen to make it possible to set either non-reversed colorregistration marks CR or reversed registration marks RCR on the screen.

In the example of the software switch mode screen P3 shown in FIG.19(C), concerning the mark detection by the registration sensors 12etc., the second mark producing method is selected. In this example, inthe software switch mode screen P3, the message “Software switch mode”,“15-1:1”, for example, as the “NO-A:B” in the registration mark settingscreen, three software switches SW1, SW2, and SW3, an “ON” key Ky1 forsettling, and an “OFF” key Ky2 for resetting are indicated.

In this screen P3, by the use of the software switches SW1 and SW2, forexample, the color registration mark selection number NO=“15” isselected. The software switch SW3 is used when either the automaticselection or the manual selection is set concerning the mark producingmethod, and when either the first or the second mark producing method isset. Automatic selection is set by the selection bit A=0, and manualselection is set by the selection bit A=1. The first mark producingmethod is set by the selection bit B=0, and the second mark producingmethod is set by the selection bit B=1.

The “ON” key Ky1 is used a setting is settled, and the “OFF” key Ky2 isused when a setting is reset. Accordingly, in this example, “15-1:1”indicates that the selection number of the color registration mark is“15”, and as regards the mark producing method, the second producingmethod has been selected by manual selection. In addition to this, asregards the detection of color registration marks CR, it is also carriedout to set by which registration sensor, 12A or 12B, the detection is tobe made, and by which mark producing method the color registration marksCR are to be formed.

FIG. 20 is a conceptual drawing showing an example of the formation ofnon-reversed and reversed color registration marks CR and RCR on theintermediate transfer belt 6.

In this example, it is practiced that non-reversed color registrationmarks CR on the intermediate transfer belt 6 are detected by theregistration sensor 12A, and reversed color registration marks RCR onthe intermediate transfer belt 6 are detected by the registration sensor12B.

In summary, non-reversed color registration marks CR and reversed colorregistration marks RCR are formed by the first or second mark producingmethod which is automatically or manually set by the software switchmode screen shown in FIG. 19(C), by the first or second mark producingmethod which is automatically selected on the basis of the new beltdetection circuit 90, or by the first or second mark producing methodwhich is automatically selected on the basis of the result of detectionof the state of use of the intermediate transfer belt 6.

On the left side part of the intermediate transfer belt 6 which isapparently traced by the registration sensor 12A, non-reversed colorregistration marks CR of the colors BK, M, etc. are formed in the sameway as a conventional method. On the right side part of the intermediatetransfer belt 6 that is apparently traced by the registration sensor12B, reversed color registration marks RCR of the color BK are formed inthe writing K. In this example, the first mark (K) and the second mark(K) are serially formed and this registration mark formation area is asignal reading area Ak.

Further, in the writing M, registration marks of the color M are formed;in this example, the first mark (M) and the second mark (M) are seriallyformed, and this registration mark formation area becomes a signalreading area Am. An area other than these reversed color registrationmarks RCR of the colors BK, M, etc., for example, an area between thesignal reading area Ak and the signal reading area Am is made to be asignal masking area Msk.

The processing for this signal masking area Msk is done in such a waythat a passage timing pulse signal Sp is masked by a mask generationcircuit 515 shown in FIG. 3 and FIG. 17. Further, in the deviationamount detection section 552 shown in FIG. 17, the relative amount ofdeviations of the first mark (M), the second mark (M), . . . , etc. arecalculated, for the purpose of superposing the component color imageswith a good reproducibility.

FIG. 21 is a perspective view showing another example of arrangement ofthe registration sensors 12A etc. In this example, with the runningdirection of the intermediate transfer belt 6 taken as the sub-scanningdirection, and the direction perpendicular to the sub-scanning directiontaken as the main scanning direction, a plurality of detection devicesare arranged in the sub-scanning direction, and the detection of thestate of the registration mark formation area of the intermediatetransfer belt 6 is separately borne by the plural detection devices.

In the example of arrangement shown in FIG. 21, three registrationsensors 12A to 12C are arranged. The registration sensor 12A is disposedin the right side with respect to the running direction of theintermediate transfer belt 6, the registration sensor 12B is disposed inthe left side, and the registration sensor 12C is disposed at thecenter, to make up a detection system based on a 3 row scheme of left,right and center. For scratches produced on the registration markformation area of the intermediate transfer belt 6 by the change withthe passage of time, a mask generation method adapted for thedistribution of the scratches can be selected.

That is, the surface state of the registration mark formation area ofthe intermediate transfer belt 6 is divided into plurality of parts ofwhich the detection can be borne separately by this arrangement ofsensors. Accordingly, for each of the divisional registration markformation areas extending to the sub-scanning direction to be detectedby the respective registration sensors 12, mark image information orreversed mark image information for forming color registration marks orreversed color registration marks can be read out selectively from thestorage device 14.

Compared to the detection system of a left-and-right two-row schemeshown in FIG. 9, the detection system of a left-center-right three-rowscheme makes it possible to set the first adjustment mode or the secondadjustment mode for each of the divisional registration mark formationareas. Accordingly, it is possible to select either the first markproducing method or the second mark producing method for each of theregistration sensors 12A, 12B, and 12C. By doing this, the toner amountof marks can be suppressed, and the influence of belt scratches on thedetection of color registration marks can be also suppressed.

FIG. 22 is a waveform drawing showing an example of signal obtained atthe time of the correction of the base level by the registration sensors12A etc. In FIG. 22, the abscissa represents time t, and the ordinaterepresents the signal level of a position detection signal S2 at thetime of the correction of the base level by the registration sensors 12Aetc. The solid line shown in FIG. 22 is a waveform expressing the stateof the registration mark formation area of the intermediate transferbelt 6 before the formation of color registration marks. The level Lbdenotes the base correction level of the position detection signal S2.Lth denotes the threshold voltage.

This waveform is obtained by the detection of the registration markformation area by the registration sensor 12A or the like through onerevolution of the intermediate transfer belt 6. This processing ofobtaining a waveform is called a base level correction. Whether or notan obstruction of the detection of color registration marks is presentjudged by whether or not a signal level lower than the threshold voltageLth is present. According to the signal example shown in FIG. 22, aposition detection signal S2 which does not come down to the thresholdvoltage Lth in spite of containing belt scratches etc. is detected. FIG.22 show the case where there are no scratches becoming the obstructionof the detection of color registration marks on the intermediatetransfer belt 6.

Subsequently, as regards the third image forming method of the presentinvention, the operation of the color image forming apparatus 300 willbe explained with reference to three examples of practice.

Example 1

FIG. 23 is a flow chart showing an example of the operation (at the timeof the detection of presence or absence of scratches) of the color imageforming apparatus 300 as the first example of the present invention.

In this example, there is provided the mark detection judgement means 80consisting of the registration sensors 12, the storage device 14, andthe control device 15 explained in FIG. 16; by the registration sensors12, the surface state of the registration mark formation area of theintermediate transfer belt 6 is detected, and in the control device 15,a detection unable flag FG is prepared. This detection unable flag FG isadopted as the basis of judgement concerning whether or not an erroneousdetection is to be made by the registration sensors 12A etc. at the timeof the detection of color registration marks.

With this taken as an operation condition, first, in order to detect thesurface state of the registration mark formation area of theintermediate transfer belt 6, a base level correction processing ispracticed in the step C1 of the flow chart shown in FIG. 23. In thisprocessing, the registration mark formation area is cleaned through thedriving of the endless-shaped intermediate transfer belt 6, and theprocedure moves to the step C2, where the sensor output of theregistration sensors 12A, 12B, etc. for one revolution of the belt areread in the RAM 57 or the like, for the purpose of sampling the noisesduring at least one revolution period of the intermediate transfer belt6. Then, the procedure moves to the step C3, where it is judged if thereis a scratch on the registration mark formation area of the intermediatetransfer belt 6.

As regards the judgement of a scratch being present or not made at thistime, before color registration marks CR are formed, if there is ascratch, a position detection signal S2 which comes down to a level muchlower than the threshold voltage Lth from the base correction level Lbas shown in FIG. 55 is detected. If the intermediate transfer belt is anew one before no color registration mark is formed, a positiondetection signal S2 of a base correction level as shown in FIG. 22 is tobe detected. If scratches are produced on the registration markformation area of the intermediate transfer belt 6 due to the changewith the passage of time, a position detection signal S2 of a levelcoming down lower than the threshold voltage Lth is detected. Thisposition detection signal S2 is binarized by the comparator 59 at thetime of the detection of color registration marks, is converted into apassage timing pulse signal Sp, and is outputted through the maskgeneration circuit 515 to the latch circuit 56.

In the case where a position detection signal S2 of a level coming downto a value lower than the threshold voltage Lth in the detection(grasping) processing of the state of use of the intermediate transferbelt 6, there is a possible risk of an erroneous detection being madeafter the formation of color registration marks. Therefore, if it isjudged that there is a scratch on the intermediate transfer belt 6 inthe step C3, the detection unable flag FG is set to establish FG=1 inthe step C4. In contrast with this, if it is judged that there is noscratch on the intermediate transfer belt 6 in the step C3, thedetection unable flag FG is reset to establish FG=0 in the step C5. Thedetection unable flag FG is temporarily memorized in the RAM 57.

(Example of Setting of Positional Deviation Adjustment Mode)

FIG. 24 is a flow chart showing an example of setting of the positionaldeviation adjustment mode in the color image forming apparatus 300.

In this example, when mark images are transferred onto the intermediatetransfer belt 6, the control device 15 judges whether or not the surfacestate of the intermediate transfer belt 6 is good, and in accordancewith this surface state of the intermediate transfer belt 6 being goodor not good, it makes a control to form color registration marks CR orreversed color registration marks RCR as the reversed ones of said colorregistration marks CR for the registration of the component color imageson said intermediate transfer belt 6. In this example, it is practicedthat the first adjustment mode or the second adjustment mode is set inaccordance with the result of the judgement of the control device 15,and after that, the first mark producing method or the second markproducing method is selected to carry out the mark detection.

As for the method of forming color registration marks CR, the first markproducing method in which the effective mark portions to make upnon-reversed color registration marks CR are toner images, and thesecond mark producing method in which areas other than the effectivemark portions to make up reversed color registration marks RCR are tonerimages are prepared. Of course, mark image information for forming colorregistration marks CR and reversed mark image data DP for formingreversed color registration marks RCR are also prepared beforehand.

With this taken as an operation condition, in the step E1 of the flowchart shown in FIG. 24, the CPU 55 reads out the detection unable flagFG corresponding to the surface state of the registration mark formationarea from the RAM 57. Then, in the step E2, it judges whether or not thedetection unable flag FG=1. If the equation FG=1 is not established, inother words, in the case where FG=0 and the surface state of theregistration mark formation area is good, the procedure moves to thestep E3, where the first adjustment mode is set, and after that, theprocedure moves to the step E6, where the first mark producing method isselected. Then, the procedure moves to the step E7.

In contrast with the above, if the detection unable flag FG=1 in thestep E2, in other words, in the case where the position detection signalS2 concerning the surface state of the registration mark formation areahas a level lower than a threshold voltage determined beforehand, theprocedure moves to the step E5, where the second adjustment mode is set.After that, procedure moves to the step E6, where the second markproducing method is selected. Then, the procedure moves to the step E7.In the step E7, positional deviation adjustment processings are carriedout on the basis of the respective adjustment modes.

For example, if the first adjustment mode is set in the above-mentionedstep E3, mark image information is read out according to the first markproducing method, and color registration marks CR based on this markimage information are formed on the intermediate transfer belt 6. Thepositions of the color registration marks formed on the intermediatetransfer belt 6 are detected by the registration sensors 12A and 12B.

On the basis of the position detection of the color registration marksCR formed on the intermediate transfer belt 6, the adjustment of theforming positions of component color images are carried out. By thecontrol device 15, on the basis of the output of the registrationsensors 12A etc., with the color registration marks for the color BKtaken as the reference, the image forming units 10C, 10M, and 10Y forthe other colors C, M, and Y are controlled. By this control, thewriting positions for the colors C, M, and Y are adjusted to come toagree with the writing position for the color BK.

Further, if the second adjustment mode is set in the step E5, reversedmark image data DP are read out according to the second mark producingmethod, and reversed color registration marks RCR based on this reversedmark image data DP are formed on the intermediate transfer belt 6 by theimage forming units 10Y, 10M, 10C, and 10K. It is practiced to adjustthe forming positions of component color images on the basis of theposition detection of the mark images defined by the void portions ofthese reversed color registration marks RCR formed on the intermediatetransfer belt 6.

(Another Example at the Time of Detecting a Scratch being Present orNot)

FIG. 25 is a flow chart showing another example of the operation (at thetime of detecting a scratch being present or not) of the color imageforming apparatus 300.

In this example, there is provided the mark detection judging meansconsisting of the registration sensors 12, the storage device 14, andthe control device 15 explained in FIG. 16; when the surface state ofthe registration mark formation area of the intermediate transfer belt 6is detected by the registration sensors 12, non-reversed colorregistration marks CR are formed and the total number of the mark edgesare compared with the design value; after that, the detection unableflag is set on the basis of the result of the comparison, and then, theadjustment mode is reconsidered.

That is, as regards non-reversed color registration marks CR, the numberof mark edges to be detected at the time of detecting the marks isdefinite beforehand according to the design value. If there is such ascratch as to induce an erroneous detection on the intermediate transferbelt 6, the number of mark edges detected does not agree with the designvalue. Owing to it, the result of the calculation of color deviationsbecomes inappropriate to make it impossible to carry out the correctionprocessing normally. This number of mark edges detected is adopted asthe basis of the judgement whether the detection is to be regarded as anerroneous one or not. Accordingly, regardless of the presence or absenceof a scratch, non-reversed color registration marks CR are formed on theintermediate transfer belt 6 by the first mark producing method.

With this taken as a prior condition, first, in order to detect thesurface state of the registration mark formation area of theintermediate transfer belt 6, the sensing process of non-reversed colorregistration marks CR is practiced in the step F1 of the flow chartshown in FIG. 25. Then, the procedure moves to the step F2, where thenumber of mark edges are compared with the design value. If the totalnumber of mark edges does not agree with the design value, the proceduremoves to the step F3. In the step F3, If the total number of mark edgesexceeds the design value is judged. If the total number of mark edgesexceeds the design value, because the surface state of the registrationmark formation area is deteriorated due to the change with the passageof time, the procedure moves to the step F4, where the detection unableflag FG is set, and FG=1 is established.

Further, if the total number of mark edges agrees with the design valuein the step F2, because the surface state of the registration markformation area is good, the procedure moves to the step F6, where thedetection unable flag FG is reset, which establishes FG=0. The detectionunable flag FG is memorized temporarily in the RAM 57. The processingafter that is carried out in such a way as to follow the example ofsetting of the positional deviation adjustment mode shown in FIG. 24.

Besides, if the total number of mark edges does not reach the designvalue in the step F3, the procedure moves to the step F5, where aprocessing for abnormal mark reading is practiced. In this processing,that the formation of non-reversed color registration marks CR is notgood, that setting of positional deviation adjustment mode from now onshould not be made, etc. are indicated on the display device. As regardsthe processing after that, the same processings as those of the flowchart shown in FIG. 24.

As explained in the above, according to the color image formingapparatus and its image forming method of the example 1 of thisinvention, there is provided the mark detection judging means consistingof the registration sensors 12, the storage device 14, and the controldevice 15, the surface state of the registration mark formation area ofthe intermediate transfer belt 6 is detected by the registration sensors12, and the detection unable flag FG is prepared in the control device15. At the time of detecting color registration mark edges, whether ornot an erroneous detection is to be made by the registration sensors 12Aetc. is judged by the detection unable flag FG.

Accordingly, in the case where the intermediate transfer belt 6 is newlyused, in the case where said intermediate transfer belt 6 has been justreplaced with a new one, etc., the forming position of component colorimages can be adjusted on the basis of the position detection of colorregistration marks CR. In the case where scratches etc. are produced onthe intermediate transfer belt 6 owing to the change with the passage oftime caused by maintenance operations, wear of parts, etc., thescratches etc. can be covered by reversed color registration marks RCR;therefore, the proper positions of mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted on the basis of a high-reliability position detection signal S2with no noise signal due to scratches etc. superposed. Further, becausecomponent color images can be exactly superposed on the intermediatetransfer belt 6, it is possible to transfer a color image on a desiredpaper sheet P at a high accuracy.

Example 2

FIG. 26 is a flow chart showing an example of the operation of the colorimage forming apparatus 300 as the second example of the presentinvention.

In this example, a fuse F for detecting a new belt is attached to theintermediate transfer belt 6, and a new belt unit detection circuit 90(NEW detection means) for melting off said fuse F is provided in thecontrol device 15; in the case where the intermediate transfer belt 6 isnewly used or in the case where said intermediate transfer belt 6 hasbeen just replaced with a new one, the fuse F is melted off. Inaccordance with the result of the judgement concerning whether the fuseF is melted off or not, the mark producing method is switched over, tocarry out the detection of registration marks.

This is based on it that the fuse F is in the through-conduction statewhen the intermediate transfer belt (the belt unit) 6 is new, and thebelt unit can be judged as one in use when the fuse F is opened. As theresult of this, image position adjusting processing is carried out inthe first adjustment mode for adjusting the forming positions ofcomponent color images on the basis of a new belt detection signal Sndobtained from this new belt unit detection circuit 90.

With this taken as an operation condition, the control device 15 readsout a new belt detection signal Snd from the new belt unit detectioncircuit 90 in the step G1 of the flow chart shown in FIG. 26. Then, itis judged in the step G2 whether or not the new belt detection signalSnd is of “L” level. If the new belt detection signal Snd is of “L”level, because the belt unit is new, the procedure moves to the step G3,where the first adjustment mode is set, and a processing for settingpredetermined data required is carried out.

After that, the procedure moves to the step G4, where the counter iscleared, and further, the procedure moves to the step G5, where theselection of the first mark producing method etc. are practiced. Then,the procedure moves to the step G6, where the melting-off processing ofthe fuse F is carried out. In this processing, the fuse F is forciblyopened by it that an externally set signal Sop is supplied to the newbelt unit detection circuit 90, the transistor TR turns on, and anexcessive electric current is made to flow through the fuse F to melt itoff, and the belt is brought in a state of use. After the fuse F isopened, a new belt detection signal of “H” level is supposed to beoutputted from the new belt unit detection circuit 90 to the controldevice 15.

In this example, it is practiced that non-reversed color registrationmarks CR are formed on the registration mark formation area of theintermediate transfer belt 6 on the basis of a new belt detection signalSnd of “L” level obtained from the new belt unit detection circuit 90.If the new belt detection signal Snd is not of “L” level in the step G2,that is, if the new belt detection signal is of “H” level and the beltunit is already in use, the procedure is completed without carrying outthe new belt processing.

Besides, as shown in the first example, in the case where the settinghas been switched over from the first adjustment mode to the secondadjustment mode, and the control device controls the image forming units10Y, 10M, 10C, and 10K on the basis of the second adjustment mode, whenthe belt unit is replaced with a new one and the new belt detectionsignal Snd becomes of “L” level, that is, “the intermediate transferbelt 6 is new” is indicated, it is practiced to switch over theprocessing automatically from the second adjustment mode to the firstadjustment mode.

As explained in the above, according to the color image formingapparatus and its image forming method of the second example of thepresent invention, the fuse F for detecting a new belt is attached tothe intermediate transfer belt 6, the control device 15 is equipped withthe new belt detection circuit 90, and when the intermediate transferbelt 6 is newly used or when the intermediate transfer belt 6 has beenjust replaced with a new one, the fuse F is melted off. In accordancewith the result of the judgement concerning whether the fuse F is meltedoff or not, the mark producing method is switched over, to carry out thedetection of registration marks.

Accordingly, in the case where the intermediate transfer belt 6 is newlyused, in the case where said intermediate transfer belt 6 has been justreplaced with a new one, etc., the forming position of component colorimages can be adjusted on the basis of the position detection of colorregistration marks CR. In the case where scratches etc. are produced onthe intermediate transfer belt 6 owing to the change with the passage oftime caused by maintenance operations, wear of parts, etc., thescratches etc. can be covered by reversed color registration marks RCR;therefore, the proper positions of mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted on the basis of a high-reliability position detection signal S2with no noise signal due to scratches etc. superposed. Further, becausecomponent color images can be exactly superposed on the intermediatetransfer belt 6, it is possible to transfer a color image on a desiredpaper sheet P at a high accuracy.

Example 3

FIG. 27 is a flow chart showing an example of the operation of the colorimage forming apparatus 300 as the third example of the presentinvention.

This example is premised on the case where automatic or manual selectionis set by the use of the operation means 18 shown in FIG. 16, and on thebasis of this, either the first adjustment mode or the second adjustmentmode is set.

With this taken as a condition of operation, the control is bifurcatedin the control device 15 through the setting of automatic or manualselection in the step H1 of the flow chart shown in FIG. 27. Automaticor manual selection is set by the operation screen P1 for key operation.If the selection bit A of the software switch SW3 is “0”, and automaticselection is set, the procedure moves to the step H2, where it is judgedby the control device 15 whether the detection unable flag FG is “1” or“0”. If the detection unable flag FG=0, the procedure moves to the stepH4, where the first adjustment mode is set, and the procedure moves tothe step H5, where the firs mark producing method is selected. Afterthat, the procedure moves to the step H14.

If the detection unable flag FG=1 in the step H2, the procedure moves tothe step H6, where the second adjustment mode is set, and the proceduremoves to the step H7, where the second mark producing method isselected. After that, the procedure moves to the step H14.

Further, in the case where the selection bit A of the software switchSW3 is “1”, and manual selection is set in the step H1, the proceduremoves to the step H8, where the selection bit B of the software switchSW3 is read. Then, the procedure moves to the step H9, where it isjudged whether the selection bit B is “1” or “0”. If the selection bit Bis “0”, the procedure moves to the step H10, where the first adjustmentmode is set, and the procedure moves to the step H11, where the firstmark producing method is selected. After that, the procedure moves tothe step H14.

If the selection bit B is “1” in the step H2, the procedure moves to thestep H12, where the second adjustment mode is set, and the proceduremoves to the step H13, where the second mark producing method isselected. After that, the procedure moves to the step H14. In the stepH14, in the same way as the second example, positional deviationprocessing is carried out on the basis of the selected adjustment mode.

For example, in the case where the first adjustment mode is set in theabove-mentioned step H4 or H9, mark image information is read outaccording to the first mark producing method in the step H5 or H11, andcolor registration marks CR based on this mark image information areformed on the intermediate transfer belt 6. The positions of the colorregistration marks CR formed on the intermediate transfer belt 6 aredetected by the registration sensors 12A and 12B. It is practiced toadjust the forming positions of component color images on the basis ofthe position detection of these color registration marks CR formed onthe intermediate transfer belt 6. With the color registration marks CRfor the color BK taken as the reference, the control device 15 controlsthe image forming units 10C, 10M, and 10Y for the other colors C, M, andY. By this control, the writing positions for the colors C, M, and Y areadjusted to come to agree with the writing position for the color BK.

Further, in the case where the second adjustment mode is set in the stepH6 or H12, reversed mark image data DP are read out according to thesecond mark producing method, and reversed color registration marks RCRbased on the reversed mark image data DP are formed on the intermediatetransfer belt 6 by the image forming units 10Y, 10M, 10C, and 10K. Onthe basis of the position detection of the mark images defined by thevoid portions of these reversed color registration marks RCR formed onthe intermediate transfer belt 6, it is practiced to adjust the formingpositions of component color images.

As explained in the above, according to the color image formingapparatus and its image forming method of the third example of thepresent invention, automatic or manual selection is set by the use ofthe operation means 18, and on the basis of this, it is practiced to seteither the first adjustment mode or the second adjustment mode.

Accordingly, in the case where the intermediate transfer belt 6 is usedfor the first time, in the case where said intermediate transfer belt 6has been just replaced with a new one, etc., the forming positions ofcomponent color images can be adjusted on the basis of the positiondetection of color registration marks CR. In the case where scratchesetc. are produced on the intermediate transfer belt 6 owing to thechange with the passage of time caused by maintenance operations, wearof parts, etc., the scratches etc. can be covered by reversed colorregistration marks RCR; therefore, the proper positions of mark imagescan be exactly detected.

Owing to this, the forming positions of component color images can beadjusted on the basis of a high-reliability position detection signal S2with no noise signal due to scratches etc. superposed. On top of it, theamount of toner used in the formation of color registration marks can besuppressed. Further, because component color images can be exactlysuperposed on the intermediate transfer belt 6, it is possible totransfer a color image on a desired paper sheet P at a high accuracy.

(4) Embodiment 4

FIG. 28 is a conceptual drawing showing an example of the structure of acolor image forming apparatus 400 as the embodiment 4 of the presentinvention.

In this example of the embodiment, the intermediate transfer belt 6 asexplained in the first to third image forming apparatus is omitted, andinstead of it, a photoreceptor belt 60 to be commonly used by the imageforming systems for the four component colors is provided, and on thisphotoreceptor belt 60, a color image is formed.

Of course, also in this example of the embodiment, there is provided acontrol device for controlling image forming units 10Y′, 10M′, 10C′, and10K′ on the basis of the output of a density detection system and aposition detection system of a color image. In this control device 15,the density of patch images for color density correction is detected bythe position detection system of color images, and a binarizationreference value for the position detection of mark images are correctedon the basis of a density detection signal of the patch images outputtedfrom said position detection system.

Further, it is put into practice that even if the condition of use ofthe photoreceptor belt 60 is changed with the passage of time owing tothe change of reflected light quantity at the photoreceptor 60, thereduction of light emission quantity of the sensors, etc., the properpositions of the mark images can be exactly detected, and the formingpositions of component color images can be adjusted at a high accuracyon the basis of high-reliability position detection signals.

The color image forming apparatus 400 shown in FIG. 28 has anotherexample of the structure of the image forming apparatus of the presentinvention, and is an apparatus for forming a color image through thesuperposition of the component color images on the photoreceptor belt 60on the basis of arbitrary image information.

In FIG. 28, the color image forming apparatus 400 is made up of an imageforming apparatus mainframe 101′ and an image reading apparatus 102. Onthe image forming apparatus mainframe 101′, the image reading apparatus102 made up of an automatic document feeder 201 and a document imagescanning exposure device 202 is mounted. A document sheet d placed onthe document table of the automatic document feeder 201 is conveyed by aconveyance means, and by means of the optical system of the documentimage scanning exposure device 202, an image on one or both sides of thedocument is subjected to scanning exposure, and is read by a line imagesensor CCD.

The analog signals obtained by the reading of the line image sensor CCDare subjected to an analog processing, A/D conversion, a shadingcorrection processing, an image compression processing, etc. in an imageprocessing section (not shown in the drawing), to become imageinformation. After that, the image information is transmitted to imagewriting sections (exposure means) 3Y, 3M, 3C, and 3K, which make up therespective image forming units.

The automatic document feeder 201, in the same way as the first to thirdimage forming apparatus, is equipped with an automatic double-sideddocument conveyance means. This automatic document feeder 201 reads thecontent of a multi-page document d fed from on the document table by asingle continuous run, and the content of the document is accumulated ina storage means (an electronic RDH function). This electronic RDHfunction is conveniently used when the content of a multi-page documentis copied by the copying function, or when a multi-page document d istransmitted by the facsimile function, for example.

The image forming apparatus mainframe 101 is what is called atandem-type color image forming apparatus, and is composed of aplurality of image forming units (image forming system) 10Y′, 10M′,10C′, and 10K′, an endless-shaped photoreceptor belt 60 as an example ofthe image forming member, a paper feed-conveyance means containing are-feed mechanism (an ADU mechanism), and a fixing device 17 for fixinga toner image.

Compared to the first to third image forming apparatus of thisinvention, the image forming unit 10Y′ for forming a component image ofthe color yellow (Y) has a photoreceptor drum and a cleaning means forthe image forming member 8Y etc. taken away to the outside of the unit,and the photoreceptor belt 60 as the image forming member is commonlyused by the four image forming systems for the respective colors. Theimage forming unit 10Y′ comprises a charging means 2Y, the exposuremeans 3Y, and a developing means 4Y which are arranged at the respectivespecified positions around the photoreceptor belt facing to it. Theimage forming unit 10M′ for forming a component image of the colormagenta (M) comprises a charging means 2M, the exposure means 3M, and adeveloping means 4M.

The image forming unit 10C′ for forming a component image of the colorcyan (C) comprises a charging means 2C, the exposure means 3C, and adeveloping means 4C. The image forming unit 10K′ for forming a componentimage of the color black (BK) comprises a charging means 2K, theexposure means 3K, and a developing means 4K.

The combinations of the charging means 2Y and the exposure means 3Y, thecharging means 2M and the exposure means 3M, the charging means 2C andthe exposure means 3C, and the charging means 2K and the exposure means3K make up latent image forming means respectively. As regards thedevelopment by means of the developing devices 4Y, 4M, 4C, or 4K, it ispracticed a reverse development process with a developing bias voltagecomposed of a direct-current voltage having the polarity the same asthat of the toners used (negative polarity in this example of theembodiment) and an alternate-current voltage superposed applied. Thephotoreceptor belt 60 is entrained about a plurality of rollers and issupported in a way to be able to revolve.

The outline of an image forming process will be explained in thefollowing. Electrostatic latent images of the respective colors areformed on the photoreceptor belt 60 by the image forming units 10Y′,10M′, 10C′, and 10K′, and these electrostatic latent images aredeveloped with the toners of the respective colors. A bias voltage (notshown in the drawing) of the polarity reverse to the toners used(positive polarity in this example of the embodiment) is applied to thephotoreceptor belt 60. A synthesized color image (color toner image) isformed on the photoreceptor belt 60. After that, the color image istransferred from the photoreceptor belt 60 to a paper sheet P.

Further, a paper sheet P contained in a paper feed cassette 20A, 20B, or20C is fed, in the same way as the first to third image formingapparatus of this invention, by a conveying-out roller 21 and feedroller 22A which are provided in each of the paper feed cassette 20A,20B, and 20C, and is conveyed through conveyance rollers 22B, 22C, and22D, a registration roller 23, etc. to a transfer roller 7A; thus, onone side (front side) of the paper sheet P, the color image istransferred.

The paper sheet P, having a color image transferred on it, is subjectedto a fixing process by the fixing device 17, and is gripped by a pair ofejection rollers 17 to be placed on an output tray 25 outside themachine. The residual toner particles remaining on the circumferentialsurface of the photoreceptor belt 60 are removed by the image formingmember cleaning means 8A, and the next image formation cycle will start.In addition, as regards the double-sided image formation processing anda paper sheet P, the explanation will be omitted because they havealready been explained in FIG. 1.

At the left of the photoreceptor belt 60 in the upstream side of theabove-mentioned cleaning means 8A, there is provided a sensor fordetecting toner image density as an example of the first detection means(hereinafter referred to as a toner image density sensor 11 simply),which detects the density of a monochromatic (toner) image formed on thephotoreceptor belt 60 to generate a density detection signal S1. Ofcourse, the position of the sensor 11 is not limited to this, and it isalso appropriate to dispose the toner image density sensor 11 andregistration sensors 12 at a position shown by the broken line in FIG.28 between the transfer roller 7A and the registration roller 23.

Located by this toner image density sensor, there are provided aplurality of sensors for detecting the positional deviation of a tonerimage as an example of the second detection means (hereinafter referredto as the registration sensors 12 simply), which detect the positions ofmark images formed on the photoreceptor belt 60 (hereinafter referred toas color registration marks CR), to generate position detection signalsS2. In the image forming apparatus mainframe 101′, there is provided acontrol device 15, which practices color registration mark detectionprocessing on the basis of a density detection signal S1 and positiondetection signals S2.

The color registration mark processing here is done in such a way thatcolor registration marks CR for the registration of component colorimages are formed on the photoreceptor belt 60, and the positions (theedge, the gravity center, or the like) of these color registration marksformed on the photoreceptor belt 60 are detected by the registrationsensors 12. This processing is practiced for the purpose of adjustingthe forming positions of component color images on the basis of thepositions of the color registration marks CR. In this example too, inthe same way as the first to third image forming apparatus of thisinvention, even if the condition of use of the photoreceptor belt 60changes with the passage of time, it is possible that the properpositions of the color registration marks CR are detected exactly, andthe forming positions of component color images are adjusted at a highaccuracy on the basis of high-reliability position detection signals S2.

FIG. 29 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of the color imageforming apparatus 400 as the embodiment 4 of the present invention. FIG.29 is an enlarged drawing of the photoreceptor belt 60 and the imageforming units 10Y′, 10M′, 10C′, and 10K′ of the color image formingapparatus 400 of FIG. 29 shown as an image transfer system I and animage forming system II respectively. In FIG. 29, the color imageforming apparatus 400 has the control device 15. The toner image densitysensor 11 is connected to the control device 15, detects the density ofa toner image (monochromatic image) formed on the photoreceptor belt 60,and outputs a density detection signal S1 to the control device 15.

To the control device 15, in addition to the toner image density sensor11, the registration sensors 12 are connected; each of these detects theposition of a toner image (monochromatic image) formed on thephotoreceptor belt 60, and outputs a position detection signal S2 to thecontrol device 15. The control device 15 is supposed to control theimage forming units 10Y′, 10M′, 10C′, and 10K′ on the basis of a densitydetection signal S1 obtained from the toner image density sensor 11 andposition detection signals S2 obtained from the registration sensors 12.In this example, corrections concerning the image forming units 10Y′,10M′, 10C′, and 10K′, etc., the adjustment of the write start positionin the main scanning and sub-scanning and the position adjustment in awriting area (skew adjustment), the correction of the main scanningwrite clock signal (lateral magnification adjustment/partial lateralmagnification adjustment), etc. are made (refer to FIG. 3).

Although it depends on the contents of the control, it is alsoappropriate to adopt such a method as to control one or all of the threeimage forming units 10Y′, 10M′, and 10C′ with the image forming unit10K′ taken as the reference; this can reduce the burden of the controldevice 15. Of course, it is also possible to incorporate thephotoreceptor belt 60 into the object of the control. In that case, itmay be appropriate to provide a zigzag run correction mechanism (notshown in the drawing) and correct a zigzag run of the photoreceptor belt60 for the correction of color deviations.

To the control device 15, the image forming units 10Y′, 10M′, 10C′, and10K′ are connected; the operation is as follows: by the image formingunit 10Y′, a toner image of the color Y is formed on the photoreceptorbelt 60 on the basis of image information for the Y-color Dy, which is acomponent of arbitrary image information Din, by the image forming unit10M′, a toner image of the color M is formed on the photoreceptor belt60 on the basis of image information for the M-color Dm, by the imageforming unit 10C′, a toner image of the color C is formed on thephotoreceptor belt 60 on the basis of image information for the C-colorDc, and by the image forming unit 10K′, a toner image of the color BK isformed on the photoreceptor belt 60 on the basis of image informationfor the BK-color Dk.

In this example, a correction means 5Y is attached to the image writingsection (exposure means) for the Y-color 3Y, and is given a function toadjust the forming position of an image of the color Y on the basis of awriting position correction signal for the Y-color Sy from the controldevice 15. In the same way, a correction means 5M is attached to theimage writing section for the M-color 3M, and is given a function toadjust the forming position of an image of the color M on the basis of awriting position correction signal for the M-color Sm from the controldevice 15.

A correction means 5C is attached to the image writing section for theC-color 3C, and is given a function to adjust the forming position of animage of the color C on the basis of a writing position correctionsignal for the C-color Sc from the control device 15. A correction means5K is attached to the image writing section for the BK-color 3K, and isgiven a function to adjust the forming position of an image of the colorBK on the basis of a writing position correction signal for the BK-colorSk from the control device 15. In this example, as regards thecalculation of the amounts of color deviations, color registration marksCR for the color BK are taken as the reference. This is done for thepurpose of adjusting the writing positions of component images of thecolors Y, M, and C to make them agree with the writing position of acomponent image of the color BK.

As regards the writing position adjustment for the color Y, as has beenexplained in the embodiment 1, the writing position of the colorregistration marks CR of the color BK and the writing position of thecolor registration marks CR of the color Y are detected, and thecorrection value is calculated from the amount of deviation obtained bythe conversion operation of the writing position of the colorregistration mark CR of the color Y into the writing position of thecolor registration mark CR of the color BK. In the same way, also asregards the writing position adjustment for the color M or C, the amountof the deviation of the writing position of the color registration marksof the color M or C from the writing position of the color registrationmarks of the color BK is detected, and from this amount of deviation,each correction value is calculated. After that, the image forming units10Y′, 10M′, and 10C′ for the colors Y, M, and C except the image formingunit 10K for the color BK are subjected to the adjustment.

For this purpose, in the image forming unit 10K for the color BK, by theoutput of toner images of the color BK only onto the photoreceptor belt60, a normal writing position adjustment in the main scanning andsub-scanning and a lateral magnification adjustment, a partial lateralmagnification adjustment, a skew adjustment, etc. in the image writingsection 3K are made. This is done for the purpose of taking the color BKas the reference in the adjustment. After that, the process is moved tothe color registration adjustment using a method of this invention, inwhich the registration adjustment for making the writing positions ofcomponent images of the colors Y, M, and C agree with the writingposition of a component image of the color BK is practiced.

Further, by the control device 15, the image forming units 10Y′, 10M′,and 10C′ are controlled to form patch marks for the color densitycorrection as an example of the aforesaid patch image on thephotoreceptor belt 60. The density of the patch marks formed on thephotoreceptor belt 60 is detected by the registration sensors 12. Afterthat, the densities of color registration marks CR for the registrationof component color images are adjusted on the basis of the densities ofthe patch marks, and the image forming units 10Y′, 10M′, 10C′, and 10K′are controlled to form color registration marks CR having their densityadjusted in the above-mentioned way on the photoreceptor belt 60 by thecontrol device 15.

The positions of color registration marks formed on the photoreceptorbelt 60 are detected by the registration sensors 12. The control device15 becomes able to correct in real time the threshold voltages Lth forthe detection of the positions of color registration marks CR on thebasis of a density detection signal S2′ of patch marks outputted fromthe registration sensor 12. The image forming units 10Y′, 10M′, 10C′,and 10K′ are subjected to a control to adjust the forming positions ofcomponent color images on the basis of the above-mentioned positions ofcolor registration marks CR.

In this example, the minimum value of a density detection signal S2′ inthe non-formation area of the patch marks and the maximum value of it inthe formation area of said patch marks outputted from the registrationsensor 12 are detected, and the average value is calculated on the basisof these minimum value and maximum value.

That is, the reason of the calculation being done in the above-mentionedway is that the relation between the output value due to the reflectionlight from a base area without the patch marks and the output value dueto the reflection light from a formation area of the patch marks beingclose to each other makes the condition for determining the thresholdvoltage of binarization worst. Therefore, by the use of valuescorresponding to the above-mentioned worst case, the average value (themedian) is derived to determine the threshold voltage of binarization.In this example, with the aforesaid average value determined to be thebinarization reference value for the detection by the registrationsensor 12, the passage timing of a color registration mark CR isdetected on the basis of the above-mentioned binarization referencevalue. The binarization reference value for the detection by theregistration sensor 12 may be a value close to the average value.

Of course, the way of position detection is not restricted to this, andalso it is appropriate that, in one and the same sequence, patch marksfor the correction of color density are formed on the photoreceptor belt60 by the control device 15, the density of the patch marks formed onthe photoreceptor belt 60 are detected by the toner image density sensor11, while the density of the patch marks formed on the photoreceptorbelt 60 is detected successively by the registration sensors 12. By thesuccessive detection of the patch marks formed in detecting the patchmarks by the registration sensors 12, the density of a colorregistration mark CR of any color to make it possible to secure the mostreliable signal level can be determined.

After that, the density of color registration marks for the registrationof component color images is adjusted on the basis of the density of thepatch marks, color registration marks CR with their density adjusted areformed on the photoreceptor belt 60, the positions of the colorregistration marks formed on the photoreceptor belt 60 are detected bythe registration sensors 12, and it is carried out a registrationadjustment processing for adjusting the forming positions of componentcolor images on the basis of the positions of the color registrationmarks CR.

In addition, as regards the example of the internal structure of thepositional deviation correction system of the control device 15, pleaserefer to FIG. 3.

FIG. 30 is a conceptual drawing showing an example of the structure ofthe image writing section for the Y-color 3Y and its correction means5Y. The image writing section for the Y-color 3Y comprises asemiconductor laser light source 31, optical systems 32 and 33, apolygonal mirror 34, a polygonal mirror motor 35, and an fθ lens 36. Inthe semiconductor laser light source 31, a laser beam is generated onthe basis of image information for the Y-color Dy. A laser beam emittedfrom the semiconductor laser light source 31 has its beam shapecorrected to a specified one by the optical systems.

This light beam is deflected in the main scanning direction by thepolygonal mirror 34. The polygonal mirror 34 is rotated by the polygonalmirror motor 35 on the basis of a Y polygon CLK from the control device15. A light beam deflected by the polygonal mirror 34 is converged onthe photoreceptor belt 60 by the fθ lens 36.

In this image writing section 3 y, there is provided the correctionmeans 5Y. The correction means 5Y comprises a lens holding mechanism 41,an fθ lens adjusting mechanism 42, an optical axis adjusting mechanism43, etc. An fθ lens 36 is attached to the lens holding mechanism 41,which is movably attached to the fθ lens adjusting mechanism 42 [and theoptical axis adjusting mechanism 43]. By the fθ lens adjusting mechanism42, the lens holding mechanism is moved to rotate the fθ lens about itsoptical axis perpendicular to the image formation surface on the basisof a position correction signal Sy (YVV).

The optical axis adjusting mechanism 43 makes an operation of adjustmentby moving the lens holding mechanism 41 in the Z-direction (thedirection of the optical axis). This mechanism 42 is embodied by the useof an actuator (a piezoelectric element), and by the control of thepitch of a full-threaded bolt. The above-mentioned mechanisms areprovided for the purpose of the adjustment of the writing position of alaser beam on the photoreceptor belt 60. As regards the other imageforming units 10M′ and 10C′, the same processing is done. By doing thisway, the positional deviations of the optical system components such asthe fθ lens 36 in the image forming units 10Y′, 10M′, 10C′, and 10K′from the optimum positions can be removed.

In this example, in order to correct the binarization reference valuefor detecting the positions of color registration marks, patch marks arepreviously formed on the photoreceptor belt 60 by means of the imageforming unit 10Y′, 10M′, 10C′, or 10K′. A binarization reference valueis a threshold level of binarization for use in the detection of thepassage timing of a color registration mark formed on the photoreceptorbelt 60.

FIG. 31 is a perspective view showing an example of the arrangement ofthe toner image density sensor 11 and the registration sensors 12A and12B. In FIG. 31, the registration sensors 12A and 12B are provided abovethe portions near both the edges of the photoreceptor belt 60. In theupstream side of the registration sensor 12A, the toner image densitysensor 11 is provided.

The positions of the toner image density sensor 11 and the registrationsensors 12A and 12B are not limited to the positions shown in FIG. 31,but it is also appropriate to dispose them at the position between thetransfer roller 7A and the registration roller 23 shown by the brokenline of FIG. 28. In the case where the sensors are disposed in thisposition, because the detection position of color registration marks CRis close to the image formation position, compared to the case wherethey are disposed close to the image forming member cleaning means 8A inthe upstream side of it with respect to the belt running direction,positional deviations can be detected earlier, which contributes greatlyto the high-speed making of the image processing.

In this example, the toner image density sensor 11 and the registrationsensor 12A are arranged serially (side by side) at the specifiedpositions with respect to the running direction of the photoreceptorbelt 60. This is done for the purpose of calibrating the registrationsensors 12 on the basis of a density detection signal S1 within onerevolution period of the photoreceptor belt 60. That is, the controldevice 15 controls, for example, the image forming unit 10K′ so as toform patch marks Pm for the color density correction, namely, patches(1) to (4) having different densities respectively on the photoreceptorbelt 60 beforehand.

Further, during one revolution of the photoreceptor belt 60, the densityof the patch marks Pm having been formed is detected by the toner imagedensity sensor 11, while the density of the patch marks formed on thephotoreceptor belt 60 is detected successively by the registrationsensors 12A etc. After that, the densities of the color registrationmarks for the registration of component color images are adjusted on thebasis of the density values of the patch marks Pm.

In addition, as regards the example of the density detection of thepatch marks Pm by the registration sensors 12A etc., please refer toFIG. 6, and as regards the example of the waveform of a densitydetection signal S2′ by the registration sensor 12A or the like, pleaserefer to FIG. 7(A) and FIG. 7(B). Further, as regards the example ofsetting a threshold voltage based on the density detection of the patchmarks Pm by the registration sensor 12A or the like, please refer toFIG. 8.

FIG. 32 is a perspective view showing an example of the detection ofcolor registration marks CR by the registration sensors 12A and 12B. InFIG. 32, after the density detection of the patch marks, the imageforming units 10Y′, 10M′, 10C′, and 10K′ are controlled to form, forexample, 7-shaped color registration marks CR having its densityadjusted during the next one revolution of the photoreceptor belt 60.The positions of the color registration marks CR formed on thephotoreceptor belt 60 are detected by the registration sensors 12A and12B. Then, the control device 15 comes to practice the colorregistration control for adjusting the forming positions of componentcolor images on the basis of the positions of the color registrationmarks CR. In addition, as regards the example of binarization ofposition detection signals S2 from the registration sensors 12A etc.,please refer to FIG. 10(A) and FIG. 10(B).

Next, a fourth image forming method will be explained with reference toan example of the operation of the color image forming apparatus 400.FIG. 33 is a flow chart showing an example of the operation of the colorimage forming apparatus 400.

This example is premised on it that the photoreceptor belt 60 isprovided in the image transfer system I, and in order that a color imagemay be formed on a sheet of paper sheet P through the superposition ofcomponent color images on the photoreceptor belt 60 on the basis ofarbitrary image information, the forming positions of the componentcolor images are adjusted on the basis of the positions of colorregistration marks CR. Further, it is taken for example the case where athreshold voltage Lth used in detecting the positions of colorregistration marks CR is corrected in real time before the adjustment ofthe forming positions of component color images. The toner image densitysensor 11 (the first detection system) and the registration sensors 12Aand 12B (the second detection system) are arranged above thecircumference of the photoreceptor belt 60, with the latter two sensors12A and 12B located at positions of the same phase next to the tonerimage density sensor 11 with respect to the running direction (advancingdirection of the belt) of the photoreceptor belt 60.

With this incorporated into the image forming condition, in the step J1to J3 of the flow chart shown in FIG. 33, the initial adjustmentconcerning the detection by the registration sensors 12A and 12B etc. iscarried out, and after that, in the steps J4 to J8, the adjustment ofwriting position is made. In this initial adjustment, an optimumthreshold voltage Lth is determined from the sensor output of thesubstrate and the sensor output of the marks for density detection.

In this example, in the step J1, patch marks Pm for color densitycorrection are formed on the photoreceptor belt 60. At this time, theimage forming unit 10Y′, 10M′, 10C′ or 10K′ forms patch marks Pm ofseveral kinds having different density values on the photoreceptor belt60 as shown in FIG. 31. After that, the procedure moves to the step J2,where the density of the patch marks Pm formed on the photoreceptor belt60 is detected by the registration sensor 12A or the like. For example,a density detection signal S2′ detected by the registration sensor 12Ais such one as shown in FIG. 6(B).

Further, the procedure moves to the step J3, where the threshold voltageLth for the position detection of the color registration mark CR iscorrected on the basis of the density detection signal S2′ of the patchmarks outputted from the registration sensor 12A. The threshold voltageLth is corrected by the operation shown in FIG. 8. At this time, in thecontrol device 15, the maximum value (MAX) of a density detection signalS2′ in the formation area of the patch marks Pm outputted from theregistration sensor 12A and the minimum value (MIN) of its non-formationarea are detected, and the average value is calculated on the basis ofthe maximum value and the minimum value of the density detection signalS2′. A density detection signal S2′ in the non-formation area of patchmarks Pm reflects the substrate of the photoreceptor belt 60.

This average value is determined to be the threshold voltage Lth for theregistration sensors 12. Further, the density values of colorregistration marks CR for the registration of component color images areadjusted on the basis of the density of the patch marks Pm. By formingthe color registration marks CR on the basis of the density of the patchmarks detected in the above, it is possible to make the density of colorregistration marks CR most suitable for the position detection by theregistration sensors 12. Further, toner consumption amount andadjustment time for the formation of color registration marks can bereduced.

Then, color registration marks CR (mark images) with their densityadjusted are formed on the photoreceptor belt 60 in the step J4. Afterthat, the positions of the color registration marks CR formed on thephotoreceptor belt 60 are detected by the registration sensors 12 in thestep J5. At this time, the passage timing of any one of the colorregistration marks is detected on the basis of the threshold voltage Lthshown in FIG. 10(A). After that, in the step J6, the amounts of colordeviations are calculated on the basis of passage timings, for thepurpose of adjusting the forming positions of component color images onthe basis of the positions of the color registration marks CR.

Then, the procedure moves to the step J7, where the amounts of colordeviations are compared with a target value. If the amounts of colordeviations are not greater than the target value, the processing iscompleted without adjusting the forming positions of component colorimages. If the amounts of color deviations exceed the target value, theprocedure moves to the step J8, where a color deviation correction ismade. In this color deviation correction, for example, in the correctionmeans for the Y-color 5Y, the fθ lens adjusting mechanism 42 is drivenon the basis of a position correction signal Sy (YVV), and the lensholding mechanism 41 is moved to rotate the fθ lens about its opticalaxis perpendicular to the image formation surface for adjustment. Bydoing this, the writing position of a laser beam on the photoreceptorbelt 60 can be adjusted.

Then, the procedure moves back to the step J4, and the above-mentionedprocessings are repeated, for the purpose of making the amounts of colordeviations zero to adjust the forming positions of component colorimages to the optimum. After that, in the same way as conventionalmethods, a color image can be formed on the photoreceptor belt 60 by theimage forming units 10Y′, 10M′, 10C′, and 10K′ with the image formingposition adjusted to the optimum.

As explained in the above, according to the color image formingapparatus 400 and the image forming method as the embodiment 4 of thepresent invention, it is put into practice that the density of patchmarks Pm for color density correction is detected by the registrationsensor 12A, and the threshold voltage Lth for detecting the positions ofthe color registration marks CR is corrected on the basis of a densitydetection signal S2′ of the patch marks Pm outputted from saidregistration sensor 12A.

Accordingly, it is possible, in accordance with the condition of use, toadjust the threshold voltage Lth for detecting the positions of colorregistration marks CR in such a way as to adapt it for the state of useof the image forming units 10Y′, 10M′, 10C′, and 10K′. On top of it,because the density of color registration marks can be optimized, a highaccuracy in the detection processing of color registration marks can besecured.

Owing to this, even if the condition of use changes with the passage oftime due to the change of reflection light quantity at the photoreceptorbelt 60, the decrease of the light emission quantity of the sensors,etc., the proper positions of color registration marks can be exactlydetected; therefore, the forming positions of component color images canbe adjusted at a high accuracy on the basis of high-reliability positiondetection signals S2. Accordingly, because component color images can beexactly superposed on the photoreceptor belt 60, it is possible totransfer a color image on a desired paper sheet P at a high accuracy.

(5) Embodiment 5

FIG. 34 is a block drawing showing an example of the structure of theimage transfer system and the image forming system of a color imageforming apparatus 500 as the embodiment 5 of the present invention.

In this example of the embodiment, it is brought into practice that, inorder that a color image may be formed through the superposition ofcomponent color images on the basis of arbitrary image information,there is provided a control device 15 for controlling image formingunits 10Y′, 10M′, 10C′, and 10K′ on the basis of reversed mark imagesobtained by the reversing of mark images for the registration ofcomponent color images, at least, reversed mark images are formedbeforehand on a photoreceptor belt 60, and after that, the formingpositions of component color images are adjusted on the basis of theposition detection of mark images defined by the void portions of thesereversed mark images. Thus, even if scratches etc. are produced on thephotoreceptor belt 60 due to the change with the passage of time causedby maintenance operations and wear of parts, a color image can betransferred onto a desired paper sheet P at a high accuracy.

The color image forming apparatus 500 shown in FIG. 34 is an apparatusfor forming a color image through the superposition of component colorimages on the basis of arbitrary image information. Said apparatus 500comprises the photoreceptor belt 60, and is given a function to form acolor image on this photoreceptor belt 60 and then transfer this colorimage onto a desired paper sheet P. Along this photoreceptor belt 60,image forming units 10Y′, 10M′, 10C′, and 10K′ are provided to practicethe formation of a color image. Registration sensors 12 as an example ofthe detection means are provided, for example, at the left of thephotoreceptor belt 60, and are given a role to detect the positions ofmark images formed on the photoreceptor belt 60.

The registration sensors 12 are connected to the control device 15, bywhich the photoreceptor belt 60 and the image forming units 10Y′, 10M′,10C′, and 10K′ are controlled on the basis of output signals of theregistration sensors 12. The control device 15 controls the imageforming units 10Y′, 10M′, 10C′, and 10K′ in such a way that at leastreversed color registration marks RCR as the reversed ones of markimages for the registration of component color images are previouslyformed on the photoreceptor belt 60, and the forming positions ofcomponent color images are adjusted on the basis of the positiondetection of mark images defined by the void portions of the reversedcolor registration marks RCR. For example, by the control device 15, onthe basis of output signals of the registration sensor 12A etc., withthe reversed color registration mark RCR for the color BK taken as thereference, the image forming units 10C′, 10M′, and 10Y′ for the othercolors C, M, and Y are controlled. By this control, the writingpositions for the colors C, M, and Y are adjusted to come to agree withthe writing position for the color BK.

A storage device 14 is connected to the control device 15, and aplurality of kinds of reversed mark image data DP for reversing markimages for the registration of component color images are stored in it.Of course, the way of handling the data is not limited to this, and alsoit is appropriate that mark image information for forming mark imagesfor the registration of component color images is stored in the storagedevice 14 beforehand, and at the time of detecting color registrationmarks, reversed mark image data DP are prepared on the basis of the markimage information and the pattern width. This is done for the purpose offorming reversed color registration marks RCR as the reversed ones ofmark images on the basis of reversed mark image data DP on thephotoreceptor belt 60.

In this example, a developing device 4Y for forming a toner image of thecolor Y on the photoreceptor belt 60 is provided in the image formingunit 10Y′, a developing device 4M′ for forming a toner image of thecolor M is provided in the image forming unit 10M, a developing device4C for forming a toner image of the color M is provided in the imageforming unit 10C′, and a developing device 4K for forming a toner imageof the color BK is provided in the image forming unit 10K′; toner imagesformed on the photoreceptor belt 60 by these developing devices 4Y, 4M,4C, and 4K on the basis of the reversed mark image data DP for thecolors Y, M, C, and BK form the reversed registration marks RCR of therespective colors, and the void portions with no toner particlesdeposited represent the mark images.

In addition, as regards items having the same name and sign as thoseexplained in the embodiment 4, the explanation will be omitted becausethey have the same function. Further, as regards the example of thestructures of non-reversed color registration marks CR and reversedcolor registration marks RCR, please refer to FIG. 13(A) and FIG. 13(B).

FIG. 35 is a conceptual drawing showing an example of formation ofreversed color registration marks RCR for the colors BK, C, M, and Y.

The reversed color registration marks RCR for the color BK (hereinafterreferred to as the BK-color reversed pattern PK simply), the reversedcolor registration marks RCR for the color C (hereinafter referred to asthe C-color reversed patterns PC simply), the reversed colorregistration marks RCR for the color M (hereinafter referred to as theM-color reversed patterns PM simply), and the reversed colorregistration marks RCR for the color Y (hereinafter referred to as theY-color reversed patterns PY simply) shown in FIG. 14 are an example ofpatterns formed serially in the sub-scanning direction on thephotoreceptor belt 60.

The case where the reversed patterns PK, PC, PM, and PY are detected bythe two registration sensors 12A and 12B is shown. The registrationsensor 12A is provided above a position near the right edge of thephotoreceptor belt 60 with respect to its running direction, and theregistration sensor 12B is provided above a position near the left edgeof the photoreceptor belt 60. The broken lines represent the apparentloci of the two registration sensors due to the revolution of thephotoreceptor belt 60.

In this example of patterns, the BK-color reversed pattern PK isintegrally formed in the main scanning direction; the two mark portionsare formed as series of void portions in a toner image that is uniformover the whole width, and are detected by the registration sensors 12Aand 12B respectively. If all the reversed patterns are formed as auniform toner image over the whole width in the main scanning directioncontaining two mark portions in the above-mentioned way, tonerconsumption increases. Therefore, it is appropriate to form a uniformtoner image in the narrow area near the mark portion, to limit thepattern width Ws like the reversed patterns PC, PM, and PY for thecolors C, M, and Y respectively. Thus, the toner consumption can besuppressed.

In the example of the reversed pattern PM for the color M, the patternwidth Ws of the reversed pattern PM is made narrower than the width ofthe mark image. In this case, the toner consumption can be suppressed tothe minimum. The reversed pattern PM has a structure such that aplurality of partial figure patterns partitioning the void portions arearranged, and is different from the reversed patterns PK, PC, and PM forthe colors BK, C, and Y respectively having a rectangular patternstructure surrounding the whole of the mark images formed of the voidportions.

Reversed mark image data DP for forming these plural kinds of reversedpattern, PY, PM, PC, and PK are stored in the storage device 14. It isappropriate to select the reversed patterns PY, PM, PC, and PK by thecontrol device 15 in accordance with the state of use of thephotoreceptor belt 60.

Further, in the case where two or more kinds of reversed colorregistration mark are formed serially in the sub-scanning direction onthe photoreceptor belt 60, the image forming units 10Y′, 10M′, 10C′, and10K′ are controlled by the control device 15 so as to make the loweredge portion of a reversed color registration mark of one kind formed onthe photoreceptor belt 60 overlap the upper edge portion of a reversedcolor registration mark of another kind, for example, by one pixel.

By doing this, as in the example of the reversed pattern PM for thecolor M and the reversed pattern PY for the color Y shown in FIG. 35,concerning the range to cover uniformly with toner particles on thephotoreceptor belt 60, it is possible to make the toner image for thecolor M overlap the toner image for the color Y. By doing this way, ontop of it that the reading of mark images by the used of hardware can beeasily limited, the density of a color image after the superposition ofthe component color images can be also confirmed.

Next, the fifth image forming method will be explained with reference toan example of the operation of the color image forming apparatus 500.FIG. 36 is a flow chart showing an example of the operation of the colorimage forming apparatus 500.

This example of the embodiment relates to a case where in order that acolor image may be formed through the superposition of the componentcolor images on the photoreceptor belt 60 on the basis of arbitraryimage information, reversed mark image data DP for reversing mark imagesfor the registration of the component color images are preparedbeforehand. The reversed mark image data DP are read out from thestorage device 14 such as a ROM. Of course, it is also appropriate toprepare reversed mark image data DP on the basis of the mark imageinformation and the pattern width at the time of forming colorregistration marks. Now, take it for instance the case where colordeviations are corrected in the order the color C, M, and Y on the basisof the color BK. The correction of color deviations are made in such away that the writing positions for the colors Y, M, and C are modifiedwith respect to the writing position for the color BK taken as thereference.

With this incorporated into the image position adjustment condition,reversed color registration marks of the concerned color RCR are formedon the photoreceptor belt 60 on the basis of reversed mark image data DPin the step K1 of the flow chart shown in FIG. 36. In this example, a BKcolor reversed pattern PK is formed at first on the photoreceptor belt60 by the image forming unit 10K′. In this example, when a toner imageis formed on the photoreceptor belt 60 on the basis of reversed markimage data DP, the portion covered by toner particles on thephotoreceptor belt 60 represents the BK-color reversed pattern PK, (thereversed color registration mark), and the void portions with no tonerparticles deposited represent the mark images. Then, it is practiced todetect the positions of the mark images defined by the void portions ofthe BK-color reversed pattern PK formed on the photoreceptor belt 60 bythe registration sensors 12A etc. in the step K2.

Further, in the step K3, it is practiced to calculate the correctionvalue for the BK color deviation on the basis of the positions of themark images defined by the void portions in the control device 15. Afterthat, the procedure moves to the step K4, whether or not a colordeviation correction is to be practiced is judged by the control device15. Whether or not a color deviation correction is practiced is judgedby comparing the deviation with a control target value determinedbeforehand.

If the color deviation amount exceeds the target value and a colordeviation correction is required, the procedure moves to the step K5,where the image writing section 3K is controlled by the control device15. At this time, in the correction means for the BK-color 5K, the fθlens adjusting mechanism 42 is driven on the basis of a positioncorrection signal Sy (YVV), and the lens holding mechanism 41 is movedto rotate the fθ lens about its optical axis perpendicular to the imageformation surface for adjustment. By doing this, the writing position ofa laser beam on the photoreceptor belt 60 can be adjusted.

If the color deviation amount is not greater than the target value andno color deviation correction is required in the step K4, the proceduremoves to the step K6, and whether or not the detection of colorregistration marks for the other colors is to be practiced is judged.Because the detection of color registration marks for the other colors,namely, for the colors C, M, and Y, is to be practiced, the proceduremoves back to the step K1.

Then, in the step K1, C-color reversed patterns PC are formed on thephotoreceptor belt 60 by the image forming unit 10C′ on the basis ofreversed mark image data DP, in the step K2, the positions of the markimages defined by the void portions in the C-color reversed patterns PCare detected by the registration sensors 12A etc.

Further, in the step K3, it is practiced to calculate the correctionvalue of the color deviation amount on the basis of the positions of themark images defined by the void portions in the control device 15. Atthis time, in the control device 15, the writing position of thereversed color registration marks RCR for the color BK and the writingposition of the reversed color registration marks RCR for the color Care detected, and the correction value is calculated from the amount ofdeviation in the case where the writing position of the reversed colorregistration marks RCR for the color C is converted into the writingposition of the reversed color registration marks RCR for the color BK.

After that, the procedure moves to the step K4, and whether or not acolor deviation correction is to be practiced is judged by the controldevice 15. Whether or not a color deviation correction is to bepracticed is judged by comparing the deviation amount with a controltarget value determined beforehand in the same way as the case of thecolor BK. If the color deviation amount exceeds the target value, and acolor deviation correction is required, the procedure moves to the stepK5, where the image writing section 3C is controlled by the controldevice 15. At this time, in the correction means 5C for the color C, thefθ lens adjusting mechanism 42 is driven on the basis of a positioncorrection signal Sy (YVV), and the lens holding mechanism 41 is movedto rotate the fθ lens about its optical axis perpendicular to the imageformation surface for adjustment. By doing this, the writing position ofa laser beam on the photoreceptor belt 60 can be adjusted.

Further, in the step K1, M color reversed patterns PM are formed on thephotoreceptor belt 60 by the image forming unit 10M′ on the basis ofreversed mark image data DP, and in the step K2, the positions of themark images defined by the void portions in the M-color reversedpatterns PM are detected by the registration sensors 12A etc.

Further, in the step K3, it is practiced to calculate the correctionvalue of deviation amount for the color M on the basis of the positionsof the mark images defined by the void portions in the control device15. At this time, in the control device 15, the writing position of thereversed color registration marks RCR for the color BK and the writingposition of the reversed color registration marks RCR for the color Mare detected, and the correction value is calculated from the amount ofdeviation in the case where the writing position of the reversed colorregistration marks RCR for the color M is converted into the writingposition of the reversed color registration marks RCR for the color BK.

After that, the procedure moves to the step K4, and whether or not acolor deviation correction is to be practiced is judged by the controldevice 15. Whether or not a color deviation correction is to bepracticed is judged by comparing the deviation amount with a controltarget value determined beforehand. If the color deviation amountexceeds the target value, and a color deviation correction is required,the procedure moves to the step K5, where the image writing section 3Mis controlled by the control device 15. At this time, in the correctionmeans for the color M 5M, the fθ lens adjusting mechanism 42 is drivenon the basis of a position correction signal Sy (YVV), and the lensholding mechanism 41 is moved to rotate the fθ lens about its opticalaxis perpendicular to the image formation surface for adjustment. Bydoing this, the writing position of a laser beam on the photoreceptorbelt 60 can be adjusted.

Further, in the step K1, Y color reversed patterns PY are formed on thephotoreceptor belt 60 by the image forming unit 10Y′ on the basis ofreversed mark image data DP, in the step K2, the position of the markimages defined by the void portions in the Y color reversed pattern PYare detected by the registration sensors 12A etc.

Further, in the step K3, it is practiced to calculate the correctionvalue of the color deviation amount for the color Y on the basis of thepositions of the mark images defined by the void portions in the controldevice 15. At this time, in the control device 15, the writing positionof the reversed color registration marks RCR for the color BK and thewriting position of the reversed color registration marks RCR for thecolor Y are detected, and the correction value is calculated from theamount of deviation in the case where the writing position of thereversed color registration marks RCR for the color Y is converted intothe writing position of the reversed color registration marks RCR forthe color BK.

After that, the procedure moves to the step K4, and whether or not acolor deviation correction is to be practiced is judged by the controldevice 15. Whether or not a color deviation correction is to bepracticed is judged by comparing the deviation amount with a controltarget value determined beforehand. If the color deviation amountexceeds the target value, and a color deviation correction is required,the procedure moves to the step K5, where the image writing section 3Yis controlled by the control device 15. At this time, in the correctionmeans for the color Y 5Y, the fθ lens adjusting mechanism 42 is drivenon the basis of a position correction signal Sy (YVV), and the lensholding mechanism 41 is moved to rotate the fθ lens about its opticalaxis perpendicular to the image formation surface for adjustment. Bydoing this, the writing position of a laser beam on the photoreceptorbelt 60 can be adjusted.

As explained in the above, according to the color image formingapparatus and the image forming method as the embodiment 5 of thepresent invention, it is practiced that, by the control device 15, aBK-color reversed pattern PK is formed beforehand on the photoreceptorbelt 60, and after that, the image writing section 3C is controlled insuch a way that the forming position of the component image of the colorC is adjusted on the basis of the positions of the mark images definedby the void portions of the C-color reversed patterns PC formed on thephotoreceptor belt 60. Also as regards the color M and Y, it ispracticed that the image writing section 3M and the image writingsection 3Y are controlled in such a way that the forming positions ofthe component images of the color M and Y are adjusted.

Accordingly, because the area except the void portions defining the markimages can be covered with reversed color registration marks (tonerimages) of the colors Y, M, C, and BK, even if scratches etc. areproduced on the photoreceptor belt 60 due to the change with the passageof time caused by maintenance operations and wear of parts, the properpositions of mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted at a high accuracy on the basis of high-reliability positiondetection signals S2 with no noise signal due to scratches etc.superposed. On top of it, for the calculation of color deviation values,the structure based on a conventional method can be used as it is withalmost no alteration. Further, the reading of portions other thanreversed color registration marks RCR is eliminated by the use ofhardware, and on top of it, toner consumption can be suppressed.Accordingly, because component color images can be superposed exactly onthe photoreceptor belt 60, a color image can be transferred onto adesired paper sheet P without being influenced by the change with thepassage of time.

(6) Embodiment 6

FIG. 37 is a block drawing showing an example of the structure of theimage transfer system and image forming system of a color image formingapparatus 600 as the embodiment 6 of the present invention.

In this example of the embodiment, there is provided a control device 15for controlling image forming units 10Y′, 10M′, 10C′, and 10K′ on thebasis of the position detection of mark images. Further, mark images orreversed registration marks RCR as the reversed ones of said mark imagesfor the registration of the component color images are formed on thephotoreceptor belt 60, and the image forming units 10Y′, 10M′, 10C′, and10K′ are controlled in such a way as to adjust the forming positions ofthe component color images on the basis of the position detection ofnon-reversed mark images or mark images defined by the void portions ofreversed color registration marks formed on the photoreceptor belt 60.

Owing to this, in the case where a new photoreceptor belt 60 is used forthe first time, or in the case where an old photoreceptor belt 60 hasbeen just replaced with a new one, the forming positions of componentcolor images can be adjusted on the basis of the position detection ofmark images. On the other hand, even if scratches etc. are produced onthe photoreceptor belt 60 due to the change with the passage of timecaused by maintenance operations, wear of parts, etc., the formingpositions of component color images can be adjusted at a high accuracyon the basis of high-reliability position detection signals S2 with nonoise signal due to scratches etc. superposed.

The color image forming apparatus 500 shown in FIG. 37 is an apparatusfor forming a color image through the superposition of the componentcolor images on the basis of arbitrary image information. Said imageforming apparatus 600 comprises the photoreceptor belt 60, and is givena function of forming a color image on this photoreceptor belt 60 andthen transferring this color image onto a desired paper sheet P. Alongthis photoreceptor belt 60, there are provided image forming units 10Y′,10M′, 10C′, and 10K′, which are given a function to form a color image.A plurality of registration sensors 12 which are an example of thedetection means are disposed above the photoreceptor belt 60, and aregiven a role to detect the positions of the mark images formed on thephotoreceptor belt 60.

The control device 15 is connected to the registration sensors 12, andis supposed to control the photoreceptor belt 60 and the image formingunits 10Y′, 10M′, 10C′, and 10K′ on the basis of output signals of theseregistration sensors 12. The control device 15, in accordance with thestate of use of the photoreceptor belt 60, at least controls the imageforming units 10Y′, 10M′, 10C′, and 10K′ to form mark images or reversedcolor registration marks as the reversed ones of said mark images forthe registration of component color images on the photoreceptor belt 60,and adjust the forming positions of component color images on the basisof the position detection of the color registration marks CR or thecolor registration marks CR defined by the void portions of the reversedcolor registration marks RCR formed on the photoreceptor belt 60.

A storage device 14 is connected to the control device 15, and is givena role to store mark image information for forming color registrationmarks CR and reversed mark image data DP for forming reversed colorregistration marks RCR. Of course, the way of handling the informationis not limited to this, and also it is appropriate that mark imageinformation for forming mark images for the registration of componentcolor images are stored in the storage device 14, and at the time of theformation of color registration marks, reversed mark image data DP areprepared on the basis of the mark image information and the patternwidth, in order to form reversed color registration marks RCR as thereversed ones of the mark images on the photoreceptor belt 60 on thebasis of the reversed mark image data DP.

In this example, with the running direction of the photoreceptor belt 60taken as the sub-scanning direction and the direction perpendicular tothe sub-scanning direction taken as the main scanning direction, theplural registration sensors 12 are arranged in the sub-scanningdirection. By doing this, the surface state of the registration markformation area of the photoreceptor belt 6 o can be detected withrespect to its plural divisional parts for the plural sensors.Accordingly, for each of the divisional registration mark formationareas extending in the sub-scanning direction to be detected by theconcerned registration sensor 12, mark image information or reversedmark image information for forming color registration marks CR orreversed color registration marks RCR can be selected from the storagedevice 14.

In this example, along the photoreceptor belt 60, a developing device 4Yfor forming a toner image of the color Y is provided in the imageforming unit 10Y′, a developing device 4Y for forming a toner image ofthe color M is provided in the image forming unit 10M′, a developingdevice 4C for forming a toner image of the color C is provided in theimage forming unit 10C′, and a developing device 4K for forming a tonerimage of the color BK is provided in the image forming unit 10K′; tonerimage portions of the colors Y, M, C, and BK formed on the photoreceptorbelt 60 make up the reversed registration marks RCR of the respectivecolors, and the void portions with no toner particles depositedrepresent the mark images. In addition, as regards items having the samename and sign as those explained in the embodiment 4, the explanationwill be omitted because they have the same function.

The control device 15 detects the surface state of the registration markformation area of the photoreceptor belt 60 by means of the registrationsensors 12, and judges whether or not the surface state of thephotoreceptor belt 60 is good on the basis of the output of theregistration sensors 12 to control the selection and readout ofinformation in the storage device 14. In this example, the registrationsensors 12, the storage device 14, and the control device 15 make up amark detection judging means 80.

In the mark detection judging means 80, the surface state of theregistration mark formation area of the photoreceptor belt 60 isdetected, and in the control device 15, a detection unable flag FG isprepared. This detection unable flag FG is used as the basis of thejudgment concerning whether or not the registration sensors 12 will makean erroneous detection at the time of the detection of colorregistration marks.

In this example, the control device 15 receives the detection output ofthe registration sensors 12, and controls the image forming units 10Y′,10M′, 10C′, and 10K′ to form color registration marks CR or reversedcolor registration marks RCR as the reversed ones of said colorregistration marks CR on the registration mark formation area inaccordance with the surface state of the photoreceptor belt 60.

Now, let it be the first adjustment mode, a mode in which it ispracticed the processing to form color registration marks CR on thephotoreceptor belt 60 on the basis of mark image information(hereinafter referred to also as the first mark producing method), andadjust the forming positions of component color images on the basis ofthe position detection of the color registration marks CR formed on thephotoreceptor belt 60. Further, let it be the second adjustment mode, amode in which it is practiced the processing to form reversed colorregistration marks RCR on the photoreceptor belt 60 on the basis ofreversed mark image data DP (hereinafter referred to also as the secondmark producing method), and adjust the forming positions of componentcolor images on the basis of the position detection of the marks imagesdefined by the void portions of the reversed color registration marksRCR formed on the photoreceptor belt 60.

To the control device 15, an operation means 18 and a display device 29are connected, and these are operated to set (select) either the firstadjustment mode or the second adjustment mode being carried out. In thecontrol device 15, it is practiced to control the photoreceptor belt 60and the image forming units 10Y′, 10M′, 10C′, and 10K′ on the basis ofthe output of the operation means 18. A setting screen at the time ofimage formation is displayed on the display device 29. A touch panelwith the operation means 18 incorporated is used for the display device29.

For example, if the surface state of the registration mark formationarea is good, or if the first adjustment mode is set, it is practicedthat color registration marks CR are formed on the photoreceptor belt 60by the first mark producing method, and the forming positions ofcomponent color images are adjusted on the basis of the positiondetection of the color registration marks CR formed on the photoreceptorbelt 60.

Further, if the surface state of the registration mark formation area isinferior to a reference value determined beforehand, of if the secondadjustment mode is set, it is practiced that reversed color registrationmarks RCR are formed on the photoreceptor belt 60 by the second markproducing method, and the forming positions of component color imagesare adjusted on the basis of the position detection of the colorregistration marks CR defined by the void portions of the reversed colorregistration marks RCR formed on the photoreceptor belt 60.

In this example, a fuse F for detecting a new belt is further providedat the photoreceptor belt 60. This fuse F is connected to adiscrimination circuit 19. The discrimination circuit 19 is provided,for example, in the control device 15, operates to output a new beltdetection signal based on the melting-off or no melting-off of the fuseF, and melts off the fuse F on the basis of an externally set signal.

In this example, the control device 15 controls the image forming units10Y, 10M, 10C, and 10K to make them form color registration marks CR orreversed color registration marks RCR as the reversed ones of said colorregistration marks CR on the registration mark formation area of thephotoreceptor belt 60 on the basis of a new belt detection signalobtained from the discrimination circuit 19 for the photoreceptor belt60.

Further, in the case where the control device 15 has already controlledthe image forming units 10Y′, 10M′, 10C′, and 10K′ on the basis of thesecond adjustment mode, if a new belt detection signal Snd obtained fromthe discrimination circuit 19 indicates that “the photoreceptor belt isnew”, the control device 15 operates to switch over the mode from thesecond to the first to control the image forming units 10Y′, 10M′, 10C′,and 10K′, for the purpose of switching over the mark producing methodfrom the second to the first.

In addition, as regards the positional deviation control system and theimage formation control system of the control device 15, please refer toFIG. 17; as regards the example of the structure of the new belt unitdetection circuit 19, please refer to FIG. 18(A) and FIG. 18(B).Further, as regards the example of the display of the operation screenP1 at the time of image formation in the display device 29, please referto FIG. 19(A) to FIG. 19(C).

FIG. 38 is a conceptual drawing showing an example of the formation ofnon-reversed color registration marks CR and reversed color registrationmarks RCR on the photoreceptor belt 60.

In this example, it is brought into practice that non-reversed colorregistration marks CR on the photoreceptor belt 60 are detected by theregistration sensor 12A, and reversed color registration marks RCR onthe photoreceptor belt 60 are detected by the registration sensor 12B.

Non-reversed color registration marks CR and reversed color registrationmarks RCR are ones based on the first or second mark producing methodwhich is automatically or manually set by the software switch modescreen shown in FIG. 19(C), are ones based on the first or second markproducing method which is automatically selected on the basis of the newbelt detection circuit 90, or ones based on the first or second markproducing method which is automatically selected on the basis of theresult of detection of the state of use of the photoreceptor belt 60.

On the left side part of the photoreceptor belt 60 that is apparentlytraced by the registration sensor 12A, non-reversed color registrationmarks CR of the colors BK, M, etc. are formed in the same way as aconventional method. On the right side part of the photoreceptor belt 60which is apparently traced by the registration sensor 12B, reversedcolor registration marks RCR of the color BK are formed in the writingK; in this example, the first mark (K) and the second mark (K) isserially formed and this registration mark formation area is a signalreading area Ak.

Further, in the writing M, registration marks of the color M are formed;in this example, the first mark (M) and the second mark (M) are seriallyformed, and this registration mark formation area is a signal readingarea Am. An area other than these reversed color registration marks RCRof the colors BK, M, etc., for example, an area between the signalreading area Ak and the signal reading area Am is made to be a signalmasking area Msk.

The processing for this signal masking area Msk is done in such a waythat a passage timing pulse signal Sp is masked by a mask generationcircuit 515 shown in FIG. 3 and FIG. 17. Further, in the deviationamount detection section 552 shown in FIG. 17, the relative amount ofdeviations of the first mark (M), the second mark (M), . . . , etc. arecalculated, for the purpose of superposing the component color imageswith a good reproducibility.

FIG. 39 is a perspective view showing another example of arrangement ofthe registration sensors 12A etc. In this example, with the runningdirection of the photoreceptor belt 60 taken as the sub-scanningdirection, and the direction perpendicular to the sub-scanning directiontaken as the main scanning direction, a plurality of detection devicesare arranged in the main scanning direction, and the detection of thestate of the registration mark formation area of the photoreceptor belt60 is separately borne by the plural detection devices.

In the example of arrangement shown in FIG. 39, three registrationsensors 12A to 12C are arranged. The registration sensor 12A is disposedin the right side with respect to the running direction of thephotoreceptor belt 60, the registration sensor 12B is disposed in theleft side, and the registration sensor 12C is disposed at the center, tomake up a detection system based on a 3-row scheme of left, right andcenter. For scratches produced on the registration mark formation areaof the photoreceptor belt 60 due to the change with the passage of time,a mask generation method corresponding to the distribution of thescratches can be selected.

That is, the surface of the registration mark formation area of thephotoreceptor belt 60 is divided into plurality of parts of which thedetection can be borne separately by this arrangement of sensors.Accordingly, for each of the divisional registration mark formationareas extending to the sub-scanning direction to be detected by therespective registration sensors 12, mark image information or reversedmark image information for forming color registration marks or reversedcolor registration marks can be read out selectively from the storagedevice 14.

Compared to the detection system of a left-and-right two-row schemeshown in FIG. 32, the detection system of a left-center-right three-rowscheme makes it possible to set the first adjustment mode or the secondadjustment mode for each of the divisional registration mark formationareas. Accordingly, it is possible to select either the first markproducing method or the second mark producing method for each of theregistration sensors 12A, 12B, and 12C. By doing this, the toner amountof marks can be suppressed, and the influence of belt scratches on thedetection of color registration marks can be also suppressed. Inaddition, as regards the example of the signal at the time of the baselevel correction by means of the registration sensors 12A etc., pleaserefer to FIG. 22.

Next, an example of the color image forming apparatus 600 concerning thesixth image forming method of the present invention will be explainedwith reference to three different examples of practice. In each example,it is taken as a prior condition that a color image is supposed to beformed on the photoreceptor belt 60 through the superposition of thecomponent color image on the basis of arbitrary image information Din.

Example 1

FIG. 40 is a flow chart showing an example of the operation (at the timeof the detection of presence or absence of scratches) of the color imageforming apparatus 600 as the first example of the present invention.

In this example, there is provided the mark detection judgement means 80made up of the registration sensors 12, the storage device 14, and thecontrol device 15 explained in FIG. 37; by the registration sensors 12,the surface state of the registration mark formation area of thephotoreceptor belt 60 is detected, and in the control device 15, adetection unable flag FG is prepared. This detection unable flag FG isadopted as the basis of judgement concerning whether or not an erroneousdetection is to be made by the registration sensors 12A etc. at the timeof the detection of color registration marks.

With this taken as an operation condition, first, in order to detect thesurface state of the registration mark formation area of thephotoreceptor belt 60, a base level correction processing is practicedin the step L1 of the flow chart shown in FIG. 40. In this processing,the registration mark formation area is cleaned through the driving ofthe endless-shaped photoreceptor belt 60, and the procedure moves to thestep L2, where the sensor output of the registration sensors 12A, 12B,etc. for one revolution of the belt are written in the RAM 57 or thelike, for the purpose of sampling the noises during at least onerevolution period of the photoreceptor belt 60. Then, the proceduremoves to the step L3, where it is judged whether or not there is ascratch on the registration mark formation area of the photoreceptorbelt 60.

As regards the judgement of a scratch being present or not made at thistime, before color registration marks CR are formed, a positiondetection signal S2 which comes down to a level much lower than thethreshold voltage Lth from the base correction level Lb as shown in FIG.55 is detected. If the photoreceptor belt was a new one before no colorregistration mark is formed, it is expected that a position detectionsignal S2 of a base correction level as shown in FIG. 22 is detected. Ifscratches are produced on the registration mark formation area of thephotoreceptor belt 60 due to the change with the passage of time, aposition detection signal S2 of a level coming down lower than thethreshold voltage Lth is detected. This position detection signal S2 isbinarized by the comparator 59 at the time of the detection of colorregistration marks, is converted into a passage timing pulse signal Sp,and is outputted through the mask generation circuit 515 to the latchcircuit 56.

In the case where a position detection signal S2 of a level coming downto a value lower than the threshold voltage Lth is detected in thedetection (grasping) processing of the state of use of the photoreceptorbelt 60, there is a possible risk of an erroneous detection being madeafter the formation of color registration marks. Therefore, if it isjudged that there is a scratch on the photoreceptor belt 60 in the stepL3, the detection unable flag FG is set to establish FG=1 in the stepL4. In contrast with this, if it is judged that there is no scratch onthe photoreceptor belt 60 in the step L3, the detection unable flag FGis reset to establish FG=0 in the step L5. The detection unable flag FGis temporarily memorized in the RAM 57.

(Example of Setting of Positional Deviation Adjustment Mode)

FIG. 41 is a flow chart showing an example of setting of the positionaldeviation adjustment mode in the color image forming apparatus 600.

In this example, when mark images are formed by the photoreceptor belt60, the control device 15 judges whether or not the surface state of thephotoreceptor belt 60 is good, and in accordance with this surface stateof the photoreceptor belt 60 being good or not, it makes a control toform color registration marks CR or reversed color registration marksRCR as the reversed ones of said color registration marks CR for theregistration of the component color images on said photoreceptor belt60. In this example, it is practiced that the first adjustment mode orthe second adjustment mode is set in accordance with the result of thejudgement of the control device 15, and after that, the first markproducing method or the second mark producing method is selected, tocarry out the mark detection.

As for the method of forming color registration marks CR, the first markproducing method in which the effective mark portions to make upnon-reversed color registration marks CR are toner images, and thesecond mark producing method in which areas other than the effectivemark portions to make up reversed color registration marks RCR are tonerimages are prepared. Of course, mark image information for forming colorregistration marks CR and reversed mark image data DP for formingreversed color registration marks RCR are also prepared beforehand.

With this taken as an operation condition, in the step M1 of the flowchart shown in FIG. 41, the CPU 55 reads out the detection unable flagFG corresponding to the surface state of the registration mark formationarea from the RAM 57. Then, in the step E2, it judges whether or not thedetection unable flag FG=1. If the equation FG=1 is not established, inother words, in the case where FG=0 and the surface state of theregistration mark formation area is good, the procedure moves to thestep M3, where the first adjustment mode is set, and after that, theprocedure moves to the step M6, where the first mark producing method isselected. Then, the procedure moves to the step M7.

In contrast with the above, if the detection unable flag FG=1 in thestep M2, in other words, in the case where the surface state of theregistration mark formation area is inferior to a threshold voltagedetermined beforehand, the procedure moves to the step M5, where thesecond adjustment mode is set. After that, the procedure moves to thestep M6, where the second mark producing method is selected. Then, theprocedure moves to the step M7. In the step M7, positional deviationadjustment processings are carried out on the basis of the respectiveadjustment modes.

For example, if the first adjustment mode is set in the above-mentionedstep M3, mark image information is read out according to the first markproducing method, and color registration marks CR based on this markimage information are formed on the photoreceptor belt 60. The positionsof the color registration marks formed on the photoreceptor belt 60 aredetected by the registration sensors 12A and 12B. On the basis of theposition detection of the color registration marks CR formed on thephotoreceptor belt 60, the adjustment of the forming positions ofcomponent color images are carried out. By the control device 15, on thebasis of the output of the registration sensors 12A etc., with the colorregistration marks for the color BK taken as the reference, the imageforming units 10C′, 10M′, and 10Y′ for the other colors C, M, and Y arecontrolled. By this control, the writing positions for the colors C, M,and Y are adjusted to come to agree with the writing position for thecolor BK.

Further, if the second adjustment mode is set in the step M5, reversedmark image data DP are read out according to the second mark producingmethod, and reversed color registration marks based on these reversedmark image data DP are formed on the photoreceptor belt 60 by the imageforming units 10Y′, 10M′, 10C′, and 10K′. It is practiced to adjust theforming positions of component color images on the basis of the positiondetection of the mark images defined by the void portions of thesereversed color registration marks RCR formed on the photoreceptor belt60.

(Another Example at the Time of Detecting Scratches being Present orNot)

FIG. 42 is a flow chart showing another example of the operation (at thetime of detecting scratches being present or not) of the color imageforming apparatus 600.

In this example, there is provided the mark detection judging means madeup of the registration sensors 12, the storage device 14, and thecontrol device 15 explained in FIG. 37; when the surface state of theregistration mark formation area of the photoreceptor belt 60 isdetected by the registration sensors 12, non-reversed color registrationmarks CR are formed and the total number of the mark edges are comparedwith the design value; after that, the detection unable flag is set onthe basis of the result of the comparison, and then, the adjustment modeis reconsidered.

That is, as regards non-reversed color registration marks CR, the numberof mark edges to be detected at the time of detecting the marks isdefinite beforehand according to the design value. If there is such ascratch as to induce an erroneous detection on the photoreceptor belt60, the number of mark edges detected does not agree with the designvalue. Owing to it, the result of the calculation of color deviationsetc. becomes inappropriate to make it impossible to carry out thecorrection processing normally. This number of mark edges detected isadopted as the basis of the judgement whether the detection is to beregarded as an erroneous one or not. Accordingly, regardless of thepresence or absence of a scratch, non-reversed color registration marksCR are formed on the photoreceptor belt 60 by the first mark producingmethod.

With this taken as a prior condition, first, in order to detect thesurface state of the registration mark formation area of thephotoreceptor belt 60, the reading processing of non-reversed colorregistration marks CR is practiced in the step N1 of the flow chartshown in FIG. 42. Then, the procedure moves to the step N2, where thenumber of mark edges are compared with the design value. If the totalnumber of mark edges does not agree with the design value, the proceduremoves to the step N3. In the step N3, it is detected if the total numberof mark edges exceeds the design value. If the total number of markedges exceeds the design value, because the surface state of theregistration mark formation area is deteriorated due to the change withthe passage of time, the procedure moves to the step N4, where thedetection unable flag FG is set to establish FG=1.

Further, if the total number of mark edges agrees with the design valuein the step N2, because the surface state of the registration markformation area is good, the procedure moves to the step N6, where thedetection unable flag FG is reset, to establish FG=0. The detectionunable flag FG is memorized temporarily in the RAM 57. The processingafter that is carried out in such a way as to follow the example ofsetting of the positional deviation adjustment mode shown in FIG. 33.

Besides, if the total number of mark edges does not reach the designvalue in the step N3, the procedure moves to the step N5, where aprocessing for abnormal mark reading is practiced. In this processing,that the formation of non-reversed color registration marks CR is notgood, that setting of positional deviation adjustment mode from now onshould not be made, etc. are indicated on the display device. As regardsthe processing after that, the same processings as those of the flowchart shown in FIG. 33.

As explained in the above, according to the color image formingapparatus and its image forming method of the example 1 of thisinvention, there is provided the mark detection judging means made up ofthe registration sensors 12, the storage device 14, and the controldevice 15, the surface state of the registration mark formation area ofthe photoreceptor belt 60 is detected by the registration sensors 12,and the detection unable flag FG is prepared in the control device 15.At the time of detecting color registration mark edges, whether or notan erroneous detection is to be made by the registration sensors 12Aetc. is judged by the detection unable flag FG.

Accordingly, in the case where the photoreceptor belt 60 is newly used,in the case where said intermediate transfer belt 6 has been justreplaced with a new one, etc., the forming positions of component colorimages can be adjusted on the basis of the position detection of colorregistration marks CR. In the case where scratches etc. are produced onthe photoreceptor belt 60 owing to the change with the passage of timecaused by maintenance operations, wear of parts, etc., the scratchesetc. can be covered by reversed color registration marks RCR; therefore,the proper positions of mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted on the basis of high-reliability position detection signals S2with no noise signal due to scratches etc. superposed. Further, becausecomponent color images can be exactly superposed on the photoreceptorbelt 60, it is possible to transfer a color image on a desired papersheet P at a high accuracy.

Example 2

FIG. 43 is a flow chart showing an example of the operation of the colorimage forming apparatus 600 as the second example of the presentinvention.

In this example, a fuse F for detecting a new belt is attached to thephotoreceptor belt 60, and a new belt unit detection circuit 90 (NEWdetection means) for melting off said fuse F is provided in the controldevice 15; in the case where the photoreceptor belt 60 is newly used orin the case where said photoreceptor belt 60 has been just replaced witha new one, the fuse F is melted off. In accordance with the result ofthe judgement concerning whether the fuse F is melted off or not, themark producing method is switched over, to carry out the detection ofregistration marks.

This is based on it that the fuse F is in the through-conduction statewhen the photoreceptor belt (the belt unit) 60 is new, and the belt unitcan be judged as one in use when the fuse F is opened. As the result ofthis, image formation processing is carried out in the first adjustmentmode for adjusting the forming positions of component color images onthe basis of a new belt detection signal Snd obtained from this new beltunit detection circuit 90.

With this taken as an operation condition, the control device 15 readsout a new belt detection signal Snd from the new belt unit detectioncircuit 90 in the step Q1 of the flow chart shown in FIG. 43. Then, itis judged in the step Q2 whether or not the new belt detection signalSnd is of “L” level. If the new belt detection signal Snd is of “L”level, because the belt unit is new, the procedure moves to the step Q3,where the first adjustment mode is set, and a processing for settingpredetermined data required is carried out.

After that, the procedure moves to the step Q4, where the counter iscleared, and further, the procedure moves to the step Q5, where theselection of the first mark producing method etc. are practiced. Then,the procedure moves to the step Q6, where the melting-off processing ofthe fuse F is carried out. In this processing, the fuse F is forciblyopened by it that an externally set signal Sop is supplied to the newbelt unit detection circuit 90, the transistor TR turns on, and anexcessive electric current is made to flow through the fuse F to melt itoff, and the belt is brought in a state of use. After the fuse F isopened, a new belt detection signal of “H” level is made to be outputtedfrom the new belt unit detection circuit 90 to the control device 15.

In this example, it is practiced that non-reversed color registrationmarks CR are formed on the registration mark formation area of thephotoreceptor belt 60 on the basis of a new belt detection signal Snd of“L” level obtained from the new belt unit detection circuit 90. If thenew belt detection signal Snd is not of “L” level in the step Q2, thatis, if the new belt detection signal is of “H” level and the belt unitis already in use, the procedure is completed without carrying out thenew belt processing.

Besides, as shown in the first example, in the case where the settinghas been switched over from the first adjustment mode to the secondadjustment mode, and the control device 15 controls the image formingunits 10Y′, 10M′, 10C′, and 10K′ on the basis of the second adjustmentmode, when the belt unit is replaced with a new one and the new beltdetection signal Snd becomes of “L” level, that is, “the photoreceptorbelt 60 is new” is indicated, it is practiced to switch over theprocessing automatically from the second adjustment mode to the firstadjustment mode.

As explained in the above, according to the color image formingapparatus and its image forming method of the second example of thepresent invention, the fuse F for detecting a new belt is attached tothe photoreceptor belt 60, the control device 15 is equipped with thenew belt detection circuit 90, and when the photoreceptor belt 60 isnewly used or when the photoreceptor belt 60 has been just replaced witha new one, the fuse F is melted off. In accordance with the result ofthe judgement concerning whether the fuse F is melted off or not, themark producing method is switched over, to carry out the detection ofregistration marks.

Accordingly, in the case where the photoreceptor belt 60 is newly used,in the case where said photoreceptor belt 60 has been just replaced witha new one, etc., the forming positions of component color images can beadjusted on the basis of the position detection of color registrationmarks CR. In the case where scratches etc. are produced on thephotoreceptor belt 60 owing to the change with the passage of timecaused by maintenance operations, wear of parts, etc., the scratchesetc. can be covered by reversed color registration marks RCR; therefore,the proper positions of mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted on the basis of high-reliability position detection signals S2with no noise signal due to scratches etc. superposed. Further, becausecomponent color images can be exactly superposed on the photoreceptorbelt 60, it is possible to transfer a color image on a desired papersheet P at a high accuracy.

Example 3

FIG. 44 is a flow chart showing an example of the operation of the colorimage forming apparatus 600 as the third example of the presentinvention.

This example is premised on the case where automatic or manual selectionis set by the use of the operation means 18 shown in FIG. 16, and on thebasis of this, either the first adjustment mode or the second adjustmentmode is set.

With this taken as a condition of operation, the control is bifurcatedin the control device 15 through the setting of automatic or manualselection in the step R1 of the flow chart shown in FIG. 44. Automaticor manual selection is set by the operation screen P1 for key operation.If the selection bit A of the software switch SW3 is “0”, to setautomatic selection, the procedure moves to the step R2, where it isjudged by the control device 15 whether the detection unable flag FG is“1” or “0”. If the detection unable flag FG=0, the procedure moves tothe step R4, where the first adjustment mode is set, and the proceduremoves to the step R5, where the firs mark producing method is selected.After that, the procedure moves to the step R14.

If the detection unable flag FG=1 in the step R2, the procedure moves tothe step R6, where the second adjustment mode is set, and the proceduremoves to the step R7, where the second mark producing method isselected. After that, the procedure moves to the step R14.

Further, in the case where the selection bit A of the software switchSW3 is “1”, to set manual selection in the step R1, the procedure movesto the step R8, where the selection bit B of the software switch SW3 isread. Then, the procedure moves to the step R9, where it is judgedwhether the selection bit B is “1” or “0”. If the selection bit B is“0”, the procedure moves to the step R10, where the first adjustmentmode is set, and the procedure moves to the step R11, where the firstmark producing method is selected. After that, the procedure moves tothe step R14.

If the selection bit B is “1” in the step R2, the procedure moves to thestep R12, where the second adjustment mode is set, and the proceduremoves to the step R13, where the second mark producing method isselected. After that, the procedure moves to the step R14. In the stepR14, in the same way as the second example, positional deviationprocessing is carried out on the basis of the selected adjustment mode.

For example, in the case where the first adjustment mode is set in theabove-mentioned step R4 or R9, mark image information is read outaccording to the first mark producing method in the step R5 or R11, andcolor registration marks CR based on this mark image information areformed on the photoreceptor belt 60. The positions of the colorregistration marks CR formed on the photoreceptor belt 60 are detectedby the registration sensors 12A and 12B. It is practiced to adjust theforming positions of component color images on the basis of the positiondetection of these color registration marks CR formed on thephotoreceptor belt 60. With the color registration marks CR for thecolor BK taken for the reference, the control device 15 controls theimage forming units 10C′, 10M′, and 10Y′, for the other colors C, M, andY. By this control, the writing positions for the colors C, M, and Y areadjusted to come to agree with the writing position for the color BK.

Further, in the case where the second adjustment mode is set in the stepR6 or R12, reversed mark image data DP are read out according to thesecond mark producing method, and reversed color registration marks RCRbased on this reversed mark image data DP are formed on thephotoreceptor belt 60 by the image forming units 10Y′, 10M′, 10C′, and10K′. On the basis of the position detection of the mark images definedby the void portions of these reversed color registration marks RCRformed on the photoreceptor belt 60, it is practiced to adjust theforming positions of component color images.

As explained in the above, according to the color image formingapparatus and its image forming method of the third example of thepresent invention, automatic or manual selection is set by the use ofthe operation means 18, and on the basis of this, it is practiced to seteither the first adjustment mode or the second adjustment mode.

Accordingly, in the case where the photoreceptor belt 60 is used for thefirst time, in the case where said photoreceptor belt 60 has been justreplaced with a new one, etc., the forming positions of component colorimages can be adjusted on the basis of the position detection of colorregistration marks CR. In the case where scratches etc. are produced onthe photoreceptor belt 60 owing to the change with the passage of timecaused by maintenance operations, wear of parts, etc., the scratchesetc. can be covered by reversed color registration marks RCR; therefore,the proper positions of mark images can be exactly detected.

Owing to this, the forming positions of component color images can beadjusted on the basis of high-reliability position detection signals S2with no noise signal due to scratches etc. superposed. On top of it, theamount of toner used in the formation of color registration marks can besuppressed. Further, because component color images can be exactlysuperposed on the photoreceptor belt 60, it is possible to transfer acolor image on a desired paper sheet P at a high accuracy.

As has been explained up to now, according to the first image formingapparatus and image forming method of the present invention, there isprovided a control device for controlling image forming means on thebasis of the output of a density detection system and a positiondetection system for toner images; this control device detects thedensity of patch images for color density correction by means of theposition detection system for monochromatic (toner) images, and correctsthe binarization reference value for the position detection of markimages on the basis of density detection signals of the patch imageoutputted from said position detection system.

By this structure, the binarization reference value for the positiondetection of mark images can be corrected in such a way as to be adaptedfor the condition of use of the image transfer means. Accordingly, evenif the condition of use changes with the passage of time caused by thechange of reflection light quantity at the intermediate transfer belt orphotoreceptor belt making up the image transfer system, the reduction oflight emission quantity of the sensors, etc., the proper formingpositions of component color images can be exactly detected; therefore,the forming positions of component color images can be adjusted at ahigh accuracy on the basis of a high-reliability position detectionsignal. Owing to this, because component color images can be superposedexactly by the image forming means, a color image can be transferredonto a desired transfer paper sheet at a high accuracy.

According to the second image forming apparatus and image forming methodof the present invention, in order that a color image may be formedthrough the superposition of the component color images on the basis ofarbitrary image information, with a control device for controlling animage forming means on the basis of reversed mark images as the reversedones of mark images for the registration of component color imagesprovided, at least, reversed mark images are formed beforehand by theimage forming means, and after that, the image forming means arecontrolled in such a way as to adjust the forming positions of componentcolor images on the basis of the position detection of the mark imagesdefined by the void portions of the reversed mark images.

By this structure, because areas other than the void portions definingmark images can be covered by the reversed mark images, even ifscratches etc. are produced on the intermediate transfer belt orphotoreceptor belt making up the image transfer system due to the changewith the passage of time caused by maintenance operations, wear ofparts, etc., the proper positions of the mark images can be exactlydetected. Accordingly, the forming positions of component color imagescan be adjusted at a high accuracy on the basis of high-reliabilityposition detection signals with no noise signal due to scratches etc.superposed. Owing to this, because component color images can besuperposed exactly on the image transfer means or image forming member,a color image can be transferred onto a desired transfer paper sheet ata high accuracy.

According to the third image forming apparatus and image forming methodof the present invention, a control device for controlling the imageforming means on the basis of the position detection of monochromaticimages is provided, and this control device forms, at least, mark imagesor reversed mark images as the reversed ones of said mark images for theregistration of component color images by the image forming means inaccordance with the state of use of the intermediate transfer belt orphotoreceptor belt, and controls the image forming means in such way asto adjust the forming positions of component color images on the basisof the position detection of the mark images or the mark images definedby the void portions of the reversed mark images formed by this imageforming means.

By this structure, in the case where the image transfer means or imageforming member making up the image transfer system is used for the firsttime, in the case where said image transfer means or image formingmember is replaced with a new one, etc., the forming positions ofcomponent color images can be adjusted on the basis of the positiondetection of mark images. In the case where scratches etc. are producedon the image transfer means or image forming member owing to the changewith the passage of time caused by maintenance operations, wear ofparts, etc., the scratches etc. can be covered by the reversed markimages; therefore, the proper positions of the mark images can beexactly detected. Accordingly, the forming positions of component colorimages can be adjusted at a high accuracy on the basis ofhigh-reliability position detection signals with no noise signal due toscratches etc. superposed.

It is extremely appropriate to apply this invention to a color printeror copying machine of a tandem type having an intermediate transfer beltor a photoreceptor belt.

(7) Embodiment 7

In the following, with reference to the drawings, FIG. 1 to FIG. 4, FIG.9, and FIG. 45 to FIG. 53, an image forming apparatus and an imageforming method of the embodiment 7 of the present invention foraccomplishing its second object will be explained.

A color image forming apparatus 100 shown in FIG. 1 as a conceptualdrawing is also an example of the structure of an image formingapparatus of the Embodiment 7.

In this example of the embodiment, in order that a color image may beformed through the superposition of component color images on the basisof arbitrary image information, a control device for controlling animage transfer means or/and an image forming units on the basis of thepositions of series of mark images for the registration of componentcolor images is provided, concerning the mark images for theregistration of component color images formed on the image transfermeans, position data representing the forming positions of the markimages with respect to a reference value determined arbitrarily areobtained, the position data of the mark images for the respective colorsare sorted into groups for their respective unit basic ranges,concerning the position data sorted for each unit basic range, anoperation processing for converting them into those based on respectivereference values representing the front edge of the unit basic rangesconcerned is carried out, the position data representing ranges commonto the mutually overlapping ranges each of which is represented by acouple of position data corresponding to the rising and falling edges ofa passage timing pulse are extracted as the position data of the markimages for the registration of component color images, and positionaldeviations of the component color images from one another are calculatedon the basis of the position data extracted in the above.

By doing this, it becomes possible to eliminate the data representing arange not overlapped by any other range represented by a pair ofposition data as position data concerning scratches. On top of it, evenin the case where position data containing data of scratches areobtained due to the use of image transfer means with the passage oftime, it is possible to extract position data not containing theinfluence of scratches for each of the colors.

FIG. 45(A) is a drawing showing an example of a color registration markCR printed, and FIG. 45(B) is a drawing showing an example of thewaveform of a position detection signal S2 by a registration sensor 12Aor the like and that of a passage timing pulse signal SP.

The color registration mark CR shown in FIG. 45(A) is an example of themark images, and with the running direction of the intermediate transferbelt 6 taken as the sub-scanning direction and the directionperpendicular to this sub-scanning direction taken as the main scanningdirection, each mark is formed of a combination of a line image parallelto the main scanning direction and an oblique line image notperpendicular to the main scanning direction. For example, as regardsthe color registration marks CR, each one of them, MARKi is formed in a7-shape. The registration marks, MARK's are formed serially one afteranother in a specified unit basic range Pr on the intermediate transferbelt 6.

The reason for the shape being made such one is that the formingpositions of marks can be detected. The forming position of a mark isdetected by the timings when the edges of a registration mark MARK passunder the registration sensor 12A or the like. In this example, theposition detection signal S2 outputted by the registration sensor 12A orthe like shown in FIG. 9 is binarized on the basis of a specifiedthreshold voltage Lth shown in FIG. 45(B).

In this example, a position detection signal S2 is rising at the timewhen it crosses the threshold line Lth at the edge (1) of a line imageparallel to the main scanning direction, and after that, the signal S2is falling at the edge (2). Subsequently, the position detection signalS2 is rising at the time when it crosses the threshold line Lth at theedge (3) of the oblique line image not perpendicular to the mainscanning direction, and after that, the signal S2 is falling at the edge(4). That is, from one 7-shaped registration mark (MARK), two pulses areobtained. These pulses makes up a passage timing pulse signal SP.

This passage timing pulse signal SP is outputted from a comparator 59shown in FIG. 3 through a mask generation circuit 515 to a latch circuit56, and is used as the reference for adjusting the positional deviationsof component color images. In this example, the amount of deviations ofthe writing positions for the colors Y, M, and C with respect to thewriting position for the color BK are calculated.

According to this method of forming color registration marks CR, it isto be considered a case where belt scratches which happen to be producedon the parts having no toner particle deposited due to the change of theintermediate transfer belt 6 with the passage of time make noises to beerroneously detected by the registration sensors 12A etc., which becomesthe cause of lowering the S/N ratio of the position detection signal S2.Therefore, by a method of this invention, it is put into practice toeliminate a noise signal due to a belt scratch by data processing.

In this example, as shown in FIG. 45(B), a unit basic term Tr isdetermined for each of the mark portions of the position detectionsignal S2 of each component color, and in this unit basic term Tr, thepulse width corresponding to the period of time of the positiondetection signal S2 being of “H” level is obtained. The pulse widthobtained in this way is the period of time from the rising edge (1) ofthe position detection signal S2 to the falling edge (2), or the periodof time from the rising edge (3) to the falling edge (4).

This period of time of “H” level represents the range where aregistration mark MARKi is present (a range where a mark is formed).This period of time of “H” level is obtained for all the registrationmarks MARKi. After that, each period of time of “H” level is convertedinto time data within one unit basic term Tr containing all registrationmarks MARKi, and the ranges having a maximum frequency of occurrence areused as position data at the time of detecting marks of the colorconcerned.

FIG. 46(A) to FIG. 46(D) are waveform drawings showing an example of theoutput of passage time data DT based on a passage timing pulse signalSP.

In this example, the passage time data DT sampled by a passage timingpulse signal SP within a unit basic term Tr correspond to (dependent on)the position data DP obtained within the unit basic range Pr on theintermediate transfer belt 6 by the measurement of the forming positionof each registration mark from the reference position. Thistime-position relationship is utilized in the detection of position dataDP.

That is, by subtracting an arbitrary reference value from the passagetime data DT, the position data DP are obtained. For the position dataDP of registration marks of each of the component colors, first,reference values for the position data DP of the respective componentcolors are determined to be the design values obtained theoretically,and the values of position data DP are reduced to those with respect tothe above-mentioned reference value concerned. Then, in order to sortthese position data DP into groups for the respective unit basic rangesPr, all the position data DP are sorted into groups for their respectiveunit basic ranges Pr. As regards the position data DP for each of thecomponent colors after being sorted, the position data DP in each unitbasic range having the above-mentioned reduced values based on thetheoretical reference values are to be converted into position databased on a reference value determined to represent the front edge of theunit basic range Pr concerned. For this purpose, all the data valuescorresponding to the registration marks of the component color concernedare divided by the value of the unit basic range Pr, to obtainremainders. The remainders have approximately the same values for eachof the unit basic ranges Pr, to become position data DP corresponding tothe positions of the registration marks in the respective unit basicranges Pr. This operation is carried out for each of the componentcolors, and position data DP with respect to the respective referencevalues are obtained for the respective component colors.

As regards the position data DP for each unit basic range Pr for each ofthe component colors obtained by the above-mentioned operation, positiondata DP in one unit basic range Pr are compared with those in the otherunit basic ranges of the same color, to find overlapping rangesrepresented by the position data. The data representing the rangescommon to the above-mentioned overlapping ranges can be regarded as theposition data DP representing the position of the color registrationmarks CR of the color concerned.

In this example, when a VTOP signal shown in FIG. 46(A), a countercircuit 55 is actuated. The VTOP signal is supplied from a CPU 55 shownin FIG. 3 to the counter circuit 54 as a reset signal. In the countercircuit 54, a system clock signal SCK shown in FIG. 46(B) is counted.

In the latch circuit 56 connected to this counter circuit 54, when aregistration mark MARKi shown in FIG. 45(A) is detected by theregistration sensor 12A or the like, count values Cout are latched onthe basis of, for example, the rising edges “0, 2, 4, 6, 8, . . . ” orthe falling edges “1, 3, 5, 7, . . . ” of a passage timing pulse signalSP concerning the registration marks MARK1, MARK2, MARK3, . . . shown inFIG. 45(C).

In this example, the count values Cout “150” for the rising edge “0” ofthe passage timing pulse signal SP at the registration mark MARK1, “180”for the falling edge “1”, “300” for the rising edge “2”, “330” for thefalling edge “3”, . . . are latched in the same way, and these countvalues Cout become the passage time data DT of the registration markMARK1 shown in FIG. 46(D). For the other registration marks MARK2,MARK3, . . . , the passage time data DT are obtained in the same way.

Accordingly, the passage time data DT of one registration mark MARK1 arecomposed of four data, two rising time data and two falling time data intotal. Supposing that color registration marks CR of one color arecomposed of registration marks MARK1 to MARK4 for i=4, 64 passage timedata DT in total for the colors Y, M, C, and K are obtained. Thesepassage time data DT are stored in a RAM 57 shown in FIG. 3.

In this example, in the CPU 55 shown in FIG. 3, it is practiced tocalculate the forming positions of color registration marks CR of therespective colors Y, M, C, and K with respect to the reference valuesfrom the passage time data DT stored in the RAM 57, for the purpose ofconverting the passage time data into the position data DP.

FIG. 47(A) to FIG. 47(C) are drawings showing an example of theprocessing of the passage time data DT concerning the forming positionsof registration marks CR of the first color to the fourth color (Y, M,C, and BK).

According to an example of the record of the passage time data DT shownin FIG. 47(A), concerning the registration mark MARK1, for example,count values Cout “150” for the No. 0 of a passage timing pulse signalSP, “180” for the No. 1, “300” for the No. 2, and “330” for the No. 3are recorded in the same way.

Concerning the passage time data DT of the registration mark MARK2,count values Cout “410” for the No. 4 of a passage timing pulse signalSP, “440” for the No. 5, “560” for the No. 6, and “580” for the No. 7are recorded in the same way. As regards the other registration marksMARK3 to MARK16, passage time data DT based on the passage timing pulsesignal SP are stored for use in obtaining position data DP.

In this example, position data DP are obtained by the followingconversion operation. Out of the count values Cout from the actuation ofthe counter circuit 54 by a VTOP signal up to the detection of the firstregistration mark MARK1, an arbitrary passage data DT is selected to setan arbitrary reference value. In this example, a passage time data “140”is selected for the arbitrary reference value. Position data DP areobtained by subtracting this passage time data “140” representing thereference value from each passage time data DT. By this operation, itbecomes possible to specify the forming position of the registrationmark MARK1 from the reference position.

According to an example of the record of position data DP shown in FIG.47(B), because the passage time data showing the reference value is“140”, concerning the registration mark MARK1, the passage time data DT“150” is converted into the position data DP=“10”, DT=“180” is convertedinto the position data DP=“40”, DT=“300” is converted into the positiondata DP=“160”, and DT=“330” is converted into the position dataDP=“190”; these position data DP showing the forming position of thisregistration mark MARK1 are recorded in the RAM 57.

Concerning the registration mark MARK2, the passage time data DT=“410”is converted into the position data DP=“270”, DT=“440” is converted intothe position data DP=“300”, DT=“560” is converted into the position dataDP=“420”, and DT=“580” is converted into the position data DP=“440”;these position data DT showing the forming position of this registrationmark MARK2 are recorded in the RAM 57. As regards the other registrationmarks MARK 3 to MARK16, the passage time data “140” showing thereference value is subtracted from each passage time data DT, and theposition data DP representing the forming positions of the registrationmarks MARK1 to MARK16 are obtained. All the position data DP are storedin the RAM 57.

In this example, the position data DP of the registration marks MARK2 toMARK16 except the position data DP of the registration mark MARK1 arefurther converted into position data DP for the comparison of positionsas described below, for the purpose of comparing the position data DP ofthe registration mark MARK1 with the position data DP of the otherregistration marks MARK2 to MARK16 on the basis of reference positionstranslated from the reference positions theoretically determined for therespective component colors by a multiple of unit basic range. Theposition data DP for the comparison of the positions are obtained forthe purpose of eliminating data due to scratches from the position dataof each registration mark MARK1.

In order to obtain the above-mentioned position data for the comparisonof the positions, the position data DP of all the registration marks CRare sorted into groups for their respective unit basic ranges Pr. Theunit basic range Pr is defined for the purpose of normalizing the rangeof comparison of a passage timing pulse signal SP. For example, the datarepresenting the length of the unit basic range Pr has been designed tobe 256 (“0” to “255”). The position data DP for the comparison of thepositions are obtained by a division operation using all the positiondata of the registration marks MARK2 to MARK16 except the MARK1, whichis taken as the reference, as the dividend and “256” as the divisor.

This division processing (operation processing) is done for the purposeof converting the position data DP into those based on the referencevalues of the respective unit basic ranges. The remainder of thisdivision operation is regarded as the position data DP for thecomparison of the positions, and the position data DP of theregistration mark MARK1 and the position data DP for the comparison ofthe positions of the registration marks MARK2 to MARK16 are recorded inthe RAM 57.

According to an example of the record of the position data for thecomparison of positions shown in FIG. 47(C), because the data indicatingthe length of the unit basic range is “256”, concerning the registrationmark MARK2, when the position data DP=270 is divided by “256”, theremainder becomes “14”. This becomes the position data for thecomparison of positions “14”. By such operations, the position data DPof each registration mark MARKi of every component color are convertedinto the position data for the comparison of positions DP.

Accordingly, the position data DP=“330” is converted into the positiondata for the comparison of positions “44”, the position data DP=“420” isconverted into the position data for the comparison of positions “164”,and the position data DP=“440” is converted into the position data forthe comparison of positions “184”; these position data for thecomparison of positions DP are recorded in the RAM 57 together with theposition data indicating the forming position of the registration markMARK1.

FIG. 48(A) and FIG. 48(B) are conceptual drawings showing an example ofthe formation of color registration marks CR of the first to fourthcolor and an example of the setting of the unit basic range Pr for thecolor registration marks CR of the first color respectively. In thisexample, the writing position of the color registration marks CR of thefirst color (for example, the color BK) shown in FIG. 48(A) isdetermined to be the first reference value, the writing position of thecolor registration marks CR of the second color (for example, the colorC) is determined to be the second reference value, the writing positionof the color registration marks CR of the third color (for example, thecolor M) is determined to be the third reference value, and the writingposition of the color registration marks CR of the fourth color (forexample, the color Y) is determined to be the fourth reference value.

The color registration marks of the first color are formed between thefirst reference value and the second reference value. The colorregistration marks of the second color are formed between the secondreference value and the third reference value. The color registrationmarks of the third color are formed between the third reference valueand the fourth reference value. The color registration marks of thefourth color are formed below the fourth reference value.

FIG. 48(B) is an enlarged drawing extracted from FIG. 48(A) for the partsurrounded by the ellipsoid. In the color registration mark CR of thefirst color shown in FIG. 48(B), with the area where each registrationmark MARKi is formed made the unit basic range Pr, in the CPU 55,concerning color registration marks for the registration of componentcolor images, it is practiced to obtain position data DP of each colorregistration mark CR with respect to the reference value for each unitbasic range.

For example, in the CPU 55, the position data DP of the colorregistration marks of the first color are sorted for each of the fourunit basic ranges Pr representing the forming positions of the colorregistration marks MARK1 to MARK4 respectively. Concerning the positiondata sorted for each unit basic range Pr in the above, as explained inFIG. 47(C), in order to convert the position data DP into those based onthe reference values of the respective unit basic ranges, a divisionprocessing is carried out. By making the data values based on therespective reference values, it becomes possible to compare the positiondata DP of the first registration mark MARK1 with the position data ofthe other registration marks MARK2 to MARK4, and by the use of theresult of this comparison, it is possible to eliminate data due toscratches etc.

FIG. 49 is a drawing showing an example of a belt scratch 9 having gotmixed in the registration mark MARK1. FIG. 50(A) to FIG. 50(E) aredrawings showing an example of the operation to eliminate scratch datadue to the belt scratch 9.

In the example shown in FIG. 49, the belt scratch 9 is detected at thesame time as the position detection of the color registration mark CR ofthe first color. In this example, the belt scratch 9 is produced at theregistration mark MARK1. Accordingly, a scratch data as shown in FIG.50(A) is contained in the position data DP of the registration mark ofthe first color MARK1. However, although the position data DP of any oneof the registration marks have approximately the same values as those ofthe other registration marks, the scratch data have values differentfrom any one of these position data DP so long as similar scratch dataare not produced in the other registration marks MARK2 to MARK4.

That is, in this example, the position data DP for each of the componentcolors representing the front and rear edges of ranges common to themutually overlapping ranges each of which is represented by a couple ofposition data corresponding to the rising edge and falling edge of apassage timing pulse in the unit basic range Pr concerned shown in FIG.50(A) to FIG. 50(D) are extracted as the position data DP of the colorregistration marks of the color concerned for the color registrationadjustment as shown in FIG. 50(E). Accordingly, the range represented bythe scratch data is not overlapped by any one of the ranges representedby pairs of position data DP which have been subjected to theabove-mentioned operation processing. Owing to this, it becomes possibleto eliminate a scratch data due to the belt scratch 9 or the like with agood reproducibility. It is put into practice to calculate the amountsof positional deviations between color registration marks of thepertinent colors on the basis of the position data DP extracted here.

FIG. 51 is a conceptual drawing showing an example of the correction ofpositional deviation in the CPU 55. According to the example ofpositional deviation shown in FIG. 51, the image writing systems for thecolors C, M, and Y are adjusted with the writing system for the color BKtaken as the reference in the same way as conventional methods.

The position data DP for the color BK, for example, are expressed as(T1B, T2B). The first data T1B indicates the passage time concerning thefirst rising edge and falling edge of the registration marks MARKi, andthe second data T2B indicates the passage time concerning the nextrising edge and falling edge. For example, in the case where the countvalue Cout denoting the first rising edge is “10”, and the count valueCout denoting the falling edge is “30”, the first data T1B is “20”. Inthe case where the count value Cout denoting the next rising edge is“100”, and the count value Cout denoting the falling edge is “120”, thesecond data T1B is “110”.

In the same way, the position data for the color Y are expressed as(T1Y, T2Y). The position data for the color M are expressed as (T1M,T2M), and the position data for the color C are expressed as (T1C, T2C)in the same way.

In this example, to take the position data DP for the color BK as thereference to adjust the writing position for the color Y, assuming thatthe position data DP for the color Y (T1Y, T2Y) represent the normalwriting position, if the registration mark MARK5 of the color Y isretarded by ε for example, position data (T1Y′, T2Y′) are detected asthe position data DP for the color Y as shown in FIG. 51. In order tocorrect this positional deviation, ε1=(T1Y′−T1Y), ε2=(T2Y′−T2Y) arecalculated. The writing timing is adjusted in such a way that theseposition deviations ε1 and ε2 are made zero.

Subsequently, the image forming method of the present invention will beexplained with reference to an example of the operation of the imageforming apparatus 100. FIG. 52 is a flow chart showing an example of theoperation of the color image forming apparatus 100. FIG. 53 is a flowchart showing an example of the acquisition of position data DP based onpassage time data DT.

In this example of the embodiment, in order that a color image may beformed through the superposition of component color images on the basisof arbitrary image information, a series of color registration marks CRfor the registration of component color images are formed for each ofthe component colors on the intermediate transfer member (the imagetransfer system) 6, with a region in which each color registration markMARKi is formed taken as a unit basic range Pr.

That is, in detecting color registration marks, a series of registrationmarks MARKi (i=1, 2 . . . ) are formed on the intermediate transfer belt6 for each of the component colors in the order BK, C, M, and Y. Thepassage timing signal for the registration marks MARKi is divided by acertain unit basic time, and passage time data DT in each unit basictime are converted into position data DP. After that, the position dataDP representing ranges common to the ranges overlapping one anotherrepresented by pairs of position data DP are extracted as the positiondata DP of the color registration marks CR of the color concerned; thus,it becomes possible to eliminate the influence of belt scratches 9 etc.

With this incorporated into the image forming condition, in the step S1of the flow chart shown in FIG. 52, color registration marks of thecolors BK, C, M, and Y are formed on the basis of the color registrationmark data for the detection of the color registration. In this example,color registration marks are simultaneously formed in each of the imageforming units 10K, 10C, 10M, and 10Y.

For example, in the image forming unit 10K, the latent image of 7-shapedmarks of the color BK is written on the photoreceptor drum 1K in theimage forming unit 10K, and is developed by a toner of the color BK toform BK-color marks PK. In the same way, in the image forming unit 10C,the latent image of 7-shaped marks of the color C is written on thephotoreceptor drum 1C in the image forming unit 10C, and is developed bya toner of the color C to form C-color marks PC.

In the image forming unit 10M, the latent image of 7-shaped marks of thecolor M is written on the photoreceptor drum 1M in the image formingunit 10M, and is developed by a toner of the color M to form M-colormarks PM. In the image forming unit 10Y, the latent image of 7-shapedmarks of the color Y is written on the photoreceptor drum 1Y in theimage forming unit 10Y, and is developed by a toner of the color Y toform Y-color marks PY.

After that, the procedure moves to the step S2, where the toner imagesof the color registration marks CR of the colors BK, C, M, and Y aresuccessively transferred from the photoreceptor drums 1K, 1C, 1M, and 1Yonto the intermediate transfer belt 6. Then, the positions of the colorregistration marks of the colors BK, C, M, and Y formed on theintermediate transfer belt 6 are detected by the registration sensors12A etc.

In this example, as shown in FIG. 45(B), a position detection signal S2is detected by the registration sensor 12A or the like, and thisposition detection signal S2 is binarized on the basis of specifiedthreshold voltage Lth. The position detection signal SP after beingbinarized becomes a passage timing pulse signal SP. This signal SP isoutputted from the comparator 59 through the mask generation circuit 515to the latch circuit 56, and is used as the reference for the adjustmentof the positional deviations of component color images.

For example, after the calling of the sub-routine shown in FIG. 53, inthe step T1 of the flow chart, the counter circuit (a timer) 54 isactuated. This actuation is done for the purpose of setting referencevalues beforehand concerning color registration marks CR for theregistration of component color images formed on the intermediatetransfer belt 6, and for the color registration marks CR formed on theintermediate transfer belt 6, obtaining position data DP representingthe forming positions of the color registration marks CR with respect tothe reference values.

Then, the procedure moves to the step T2, where it is judged whether ornot an end mark is detected by the CPU 55. In this example, the end markis the last registration mark MARK16 in the Y-color mark PY. If this endmark has not been detected, the procedure moves to the step T3, where itis checked if the rising edge of the passage timing pulse signal SP atthe registration mark MARKi of the color concerned has been detected.

If the rising edge of the passage timing pulse signal SP has beendetected, the procedure moves to the step T4, where passage time data DTare stored. In this example, it is practiced that, as shown in FIG.45(B), first, at the rising edge (1) of the passage timing pulse signalSP for the color BK, a count value Cout by the counter circuit 54 islatched, and the passage time data DT are memorized (loaded) in the RAM57.

Then, the procedure moves to the step T5, where it is checked if thefalling edge of said registration mark MARKi has been detected. If thefalling edge has been detected, the procedure moves to the step T6,where the passage time data DT are memorized. In this example, it ispracticed that, as shown in FIG. 45(B), at the falling edge (2) of thepassage timing pulse signal SP for the color BK, a count value Cout fromthe counter circuit 54 is latched, and the passage time data DT arememorized (loaded) in the RAM 57.

After that, the procedure returns to the step T2. If the end mark hasnot been detected in the step T2, the procedure moves to the step T3,and the above-mentioned acquisition processing of passage time data DTis repeated. For example, it is practiced that, at the rising edge (3)of the passage timing pulse signal SP for the color BK, a count valueCout by the counter circuit 54 is latched, and the passage time data DTare memorized (loaded) in the RAM 57.

After that, it is practiced that a count value Cout by the countercircuit 54 is latched at the falling edge (4) of the passage timingpulse signal SP in the step T6, and the passage time data DT arememorized (loaded) in the RAM 57. By doing this, the position dataconcerning one registration mark MARK1 of the color BK are obtained. Inthis way, when all the forming positions of the color registration marksCR of the colors BK, C, M, and Y formed on the intermediate transferbelt 6 are detected and the end mark is detected in the step T2, theprocedure moves to the step T7.

In the step T7, the passage time data DT are converted into positiondata DP. In this example, as explained in FIG. 47(B), as regards thepassage time data DT of the color registration marks CR of each colorobtained in the steps T2 to T6, a passage time data “140” is selectedfor the arbitrary reference value. The position data DP are to beobtained by, first, subtracting this passage time data “140”representing the reference value from each passage time data DT. By thisoperation, the forming positions of the registration mark MARK1 from thereference position becomes able to be specified, and position data DPwith respect to the reference value can be obtained. As regards theother registration marks MARK2 to MARK16, the same operation is carriedout.

Then, the position data DP are sorted for each unit basic range Pr inthe step T71, and the procedure moves to the step T72, where anoperation for converting the values of the position data in each unitbasic range into values based on the reference value representing thefront edge (border between the two unit basic ranges) of the unit basicrange concerned is carried out. At this time, as explained in FIG.47(C), the position data of the registration marks MARK2 to MARK16except the registration mark MARK1 are converted into the position dataDP for the comparison of positions.

The above-mentioned operation is done for the purpose of comparing theposition data DP of the registration mark MARK1 with the position dataDP of the other registration marks MARK2 to MARK16 through thetranslation of the reference position to the front edge position of eachunit basic range. The position data DP for the comparison of thepositions are used to eliminate data due to scratches from theregistration marks MARKi.

Then, the procedure moves to the step T8, where the position data DPrepresenting ranges common to the ranges overlapping one anotherrepresented by pairs of position data DP are extracted as the positiondata DP of the color registration marks of the color concerned for theregistration of component color images (refer to FIG. 50(A) to FIG.50(E)). The position data DP extracted here are used later for thepurpose of calculating the amounts of positional deviations of componentcolor images from one another. In this way, when all the position dataDP of the color registration marks CR of the colors BK, C, M, and Yformed on the intermediate transfer belt 6 are obtained, registrationmark detection processing is completed, and the procedure moves back tothe step S3 of the main routine shown in FIG. 52.

Further, the procedure moves to the step S4, where the correction valuesof the positional deviations for the color Y are calculated on the basisof the position data DP of the color registration marks CR. In thisexample, the amounts of deviations 8 of the writing positions for thecolors Y, M, C from the writing position for the color BK arecalculated. For example, as shown in FIG. 51, to take the position dataDP for the color BK taken as the reference, and assuming that thewriting position for the color Y is to be adjusted, if the registrationmark MARK5 of the color Y is retarded by ε, the data (T1Y′, T2Y′) aredetected as the position data DP′ for the color Y. In order to correctthis positional deviation, ε1=(T1Y′−T1Y), and ε2=(T2Y′−T2Y) arecalculated.

After that, the procedure moves to the step S5, where it is judged bythe CPU 55 whether or not the color deviation correction for the colorsK, C, M, and Y is to be practiced. Whether or not color deviationcorrection is to be practiced is judged by comparing the amounts of thepositional deviations with a control target value determined beforehand.For example, if the amount of positional deviation of the color Yexceeds the target value and a correction of the color deviation isrequired, the procedure moves to the step S6, where the image writingsection 3Y is controlled by the CPU 55.

At this time, it is practiced in the correction means for the color Ythat the fθ lens adjusting mechanism 42 is driven on the basis of theposition correction signal Sy (YVV), and the lens holding mechanism 41is moved to rotate the fθ lens about its optical axis. By doing this, itis possible that the writing position of the light beam on thephotoreceptor drum 1Y is adjusted, and the writing timing is adjusted insuch a way that the above-mentioned positional deviations 81, 82, etc.become zero.

After that, the procedure moves to the step S7. In the step S7, it isjudged whether or not writing position adjustment processing is to becarried out for the other colors too. If writing position adjustmentprocessing is to be carried out for the other colors, namely, the colorsM, C, and BK too, the procedure moves back to the step S6, where theabove-mentioned processing is repeated. Further, if the amounts ofpositional deviations for all the colors BK, C, M, and Y are not greaterthan the target value to make color deviation correction unnecessary,the writing position adjustment processing is completed.

As explained in the above, according to the color image formingapparatus and the image forming method as the embodiment of thisinvention, in order that a color image may be formed through thesuperposition of the component color images on the basis of arbitraryimage information, the position data DP of a series of colorregistration marks CR of each of the component colors are sorted foreach unit basic range Pr, concerning the position data DP sorted foreach unit basic range Pr in the above, an operation for converting thevalues of the position data in each unit basic range into values basedon the reference value representing the front edge (border between thetwo unit basic ranges) of the unit basic range Pr concerned is carriedout, and the position data DP for each of the component colorsrepresenting the front and rear edges of ranges common to the mutuallyoverlapping ranges each of which is represented by a couple of positiondata corresponding to the rising edge and falling edge of a passagetiming pulse in the unit basic range Pr concerned are extracted as theposition data DP of the color registration marks for the registration ofthe component color images for each of the component colors.

Accordingly, the position data DP which represent a range not overlappedby any other range of positions represented by other position data DPcan be eliminated as position data concerning the belt scratch 9 or thelike. By doing this, even if position data DP containing those of beltscratch 9 etc. due to the use during the passage of time of theintermediate transfer belt 6 are obtained, position data DP notinfluenced by the belt scratch 9 etc. for each of the component colorscan be extracted. Owing to this, it is not required to take the troubleof forming color registration marks CR avoiding the belt scratch 9 etc.,and even if a plurality of belt scratches are produced on theintermediate transfer belt 6, position data DP not influenced by thebelt scratches 9 etc. for each of the component colors can be extracted.

Owing to this, the forming positions of component color images can beadjusted at a high accuracy on the basis of high-reliability positiondata DP with no noise signal due to the belt scratch 9 etc. superposed.Accordingly, because component color images can be exactly superposed onthe intermediate transfer belt 6, it is possible to transfer a colorimage onto a desired transfer paper sheet.

Furthermore, for the position data DP of series of color registrationmarks CR of the plural component colors formed on the intermediatetransfer belt 6, it is possible to practice the scratch eliminationprocessing in one time; therefore, it is unnecessary to practice thecolor registration mark formation sequence several times, which makes itpossible, compared to a conventional method, to shorten the calculationtime of the amounts of positional deviations.

As explained up to now, according to an image forming apparatus of theembodiment 7 of the present invention, the image forming apparatus isequipped with a control device for controlling, in order that a colorimage may be formed through the superposition of the component colorimages on the basis of arbitrary image information, an image transfermeans or/and image forming units on the basis of the position detectionof a plurality of series of mark images for the registration of thecomponent color images, this control device obtains position datarepresenting the forming positions of each series of mark images withrespect to a reference value arbitrarily determined for the mark imagesfor the registration of the component color images formed on the imagetransfer means, the position data of each series of the mark images foreach component color are sorted for each unit basic range, concerningthe position data sorted for each unit basic range in the above, anoperation processing for converting the values of the position data ineach unit basic range into values based on the reference valuerepresenting the front edge (border between the two unit basic ranges)of the unit basic range concerned, the position data DP for each of thecomponent colors representing the front and rear edges of ranges commonto the mutually overlapping ranges each of which is represented by acouple of position data corresponding to the rising edge and fallingedge of a passage timing pulse in the unit basic range Pr concerned areextracted as the position data DP of the mark images for theregistration of the component color images for each of the componentcolors, and the amounts of positional deviations of the component colorimages from one another are calculated on the basis of the position dataextracted in the above.

By this structure, the position data which represent a range notoverlapped by any other range of positions represented by other positiondata can be eliminated as position data concerning scratches.Accordingly, even if position data containing those of scratches due tothe use during the passage of time of the image transfer means areobtained position data not influenced by scratches for each of thecomponent colors can be extracted. Owing to this, it is not required totake the trouble of forming mark images avoiding a scratch, and even ifa plurality of scratches are produced on the image transfer means,position data not influenced by the scratches for each of the componentcolors can be extracted.

Furthermore, for the position data of a plurality of series of colorregistration marks CR of the plural component colors formed on the imagetransfer means, it is possible to practice the scratch eliminationprocessing in one time; therefore, it is unnecessary to practice thecolor registration formation sequence several times, which makes itpossible, compared to a conventional method, to shorten the calculationtime of the amounts of positional deviations.

According to an image forming method of the embodiment 7 of the presentinvention, in order that a color image may be formed through thesuperposition of the component color images on the basis of arbitraryimage information, position data with respect to a reference valuearbitrarily determined concerning mark images for the registration ofcomponent color images formed on an image transfer system are obtained,the position data of the mark images of each component color are sortedfor each unit basic range, an operation processing for converting thevalues of the position data in each unit basic range into values basedon the reference value representing the front edge (border between thetwo unit basic ranges) of the unit basic range concerned, the positiondata for each of the component colors representing the front and rearedges of ranges common to the plural ranges represented by pairs ofposition data overlapping one another in the unit basic range Pr areextracted as the position data of the mark images for the registrationof the component color images for each of the component colors, and theamounts of positional deviations of component color images from oneanother are calculated on the basis of the position data extracted inthe above.

By this structure, the position data which represent a range notoverlapped by any other range of positions represented by other positiondata can be eliminated as position data concerning scratches.Accordingly, even if position data containing those of scratches due tothe use during the passage of time of the image transfer system areobtained, position data not influenced by scratches for each of thecomponent colors can be extracted.

Furthermore, for the position data of a plurality of series of markimages of the plural component colors formed on the image transfersystem, it is possible to practice the scratch elimination processing inone time; therefore, it is unnecessary to practice the mark imageformation sequence a plurality of times, which makes it possible,compared to a conventional method, to shorten the calculation time ofthe amounts of positional deviations.

It is extremely suitable to apply this invention to a printer or copyingmachine of a tandem type, or a complex machine of these.

1. An image forming apparatus for forming a color image based onarbitrary image information of a color image, the apparatus comprising:(a) an image forming device having an image forming member for formingthe color image through a superposition of component color images basedon the arbitrary image information; (b) a detector for detecting aposition of the color image formed by the image forming device; and (c)a controller for controlling the image forming device based on an outputof the detector, wherein the controller forms beforehand at leastreversed mark images which are reversed from mark images for aregistration of component color images formed by the image formingdevice, and controls the image forming device to adjust formingpositions of component color images based on a position detection of themark images defined by void portions which are not filled with colortoner particles, of the reversed mark images formed by the image formingdevice.
 2. The image forming apparatus of claim 1, wherein the imageforming device comprises: a plurality of image forming units each havingan image forming member for a component color for forming a toner imagecorresponding to the component color on the image forming member; and animage transfer device to which the toner images formed on a plurality ofimage forming bodies are transferred.
 3. The image forming apparatus ofclaim 2, wherein the image transfer device comprises an intermediatetransfer body, and each of the mark images is formed by a combination ofa line image parallel to a main scanning direction and an oblique lineimage which is not perpendicular to a sub-scanning direction, where thesub-scanning direction represents a traveling direction of theintermediate transfer body and the main scanning direction represents adirection perpendicular to the sub-scanning direction.
 4. The imageforming apparatus of claim 2, wherein each of the image forming unitscomprises a developing device for forming a toner image on theintermediate transfer body, the reversed mark images comprise tonerimage portions of the intermediate transfer body formed by thedeveloping device, and the mark images comprise the void portions whichare not filled with toner images.
 5. The image forming apparatus ofclaim 2, wherein the detector outputs a positional detection signal ofthe mark images defined by the void portions of the reversed markimages, and portions other than the void portions with respect to thepositional detection signal are masked.
 6. The image forming apparatusof claim 2, wherein each of the reversed mark images is formed so as tohave arbitrary widths in a main scanning direction and a sub-scanningdirection of the intermediate transfer body, and the widths arearbitrarily changeable in the main scanning and sub-scanning directions.7. The image forming apparatus of claim 2, wherein when two or more ofthe reversed mark images are formed in a line in the main scanningdirection on the intermediate transfer body, the controller controlseach of the image forming units to form the reversed mark images in aline so that a leading edge of one of the reversed mark images formed onthe intermediate transfer body is overlapped with a trailing edge of theother of the reversed mark images in the sub-scanning direction.
 8. Theimage forming apparatus of claim 1, wherein the image forming devicecomprises: a single image forming member on which the component colorimages are superposed to form the color image; and a plurality of imageforming units each of which forms a component color image on the singleimage forming member.
 9. The image forming apparatus of claim 8, whereinthe image forming member comprises an endless body, and each of the markimages is formed by a combination of a line image parallel to a mainscanning direction and an oblique line image which is not perpendicularto a sub-scanning direction, where the sub-scanning direction representsa traveling direction of the image forming member and the main scanningdirection represents a direction perpendicular to the sub-scanningdirection.
 10. The image forming apparatus of claim 8, wherein each ofthe image forming units comprises a developing device for forming atoner image on the image forming member, the reversed mark imagescomprise toner image portions of the image forming member formed by thedeveloping device, and the mark images comprise the void portions whichare not filled with toner images.
 11. The image forming apparatus ofclaim 8, wherein the detector outputs a positional detection signal ofthe mark images defined by the void portions of the reversed markimages, and portions other than the void portions with respect to thepositional detection signal are masked.
 12. The image forming apparatusof claim 8, wherein each of the reversed mark images is formed so as tohave arbitrary widths in a main scanning direction and a sub-scanningdirection of the image forming member, and the widths are arbitrarilychangeable in the main scanning and sub-scanning directions.
 13. Theimage forming apparatus of claim 8, wherein when two or more of thereversed mark images are formed in a line in the main scanning directionon the image forming member, the controller controls each of the imageforming units to form the reversed mark images in a line so that aleading edge of one of the reversed mark images formed on the imageforming member is overlapped with a trailing edge of the other of thereversed mark images in the sub-scanning direction.